UNIVERSITY  OF  CALIFORNIA 
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THE  INTERNATIONAL  SCIENTIFIC  SERIES. 
VOLUME  VI. 


THE 

INTERNATIONAL  SCIENTIFIC  SERIES. 


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D.  APPLETON   A1SD   COMPANY,  NEW  YORK. 


THE  INTERNATIONAL  SCIENTIFIC   SERIES. 


THE 


NEW    CHEMISTRY. 


BY 

JOSIAH    PARSONS    CpOKE,    LL.  T)., 

ERVING   PROFESSOR   OF   CHEMISTRY  AND   MINERALOGY 
IN   HARVARD   UNIVERSITY. 


REVISED  EDITION,  REMODELED  AND  ENLARGED. 


NEW  YORK: 
D.  APPLETON  AND  COMPANY, 

72    FIFTH    AVENUE. 
1901. 

\ 


COPYRIGHT  BY 

D.  APPLETON   AND    COMPANY, 

1873,  1884. 


THIS    EEMODELED   WORK 
I    DEDICATE   TO 

HER 

WHOSE     AFFECTIONATE     SYMPATHY 

HAS    GLADDENED    MY   LIFE 
AND   INSPIRED    MY    RIPER    STUDIES. 


PREFACE  TO  REVISED  EDITION. 


THE  progress  in  chemistry  during  the  ten  years 
which  have  elapsed  since  this  work  was  first  published 
and  stereotyped  has  been  accompanied  by  no  such  revo- 
lution in  its  philosophy  as  the  previous  transition  from 
the  dualistic  system  of  Berzelius  to  the  unitary  system 
of  structural  organic  chemistry  had  involved.  Never- 
theless, there  has  been  a  constant  advance,  during  which 
we  have  gained  clearer  conceptions  and  more  com- 
prehensive views  of  the  fundamental  principles  of  the 
science ;  and  many  of  the  accidental  features  which 
marked  the  transition  period  have  disappeared.  Mean- 
while the  distinction  between  elementary  substances 
and  materials  consisting  of  isolated  elementary  atoms 
has  become  clear,  and  in  making  these  last,  alone,  the 
elements  of  chemistry  we  have  pushed  our  science,  if 
not  to  its  extreme  limits,  still  one  step  further  back; 
and  in  taking  this  step  we  have  left  behind  many  of 
the  anomalies  which  previously  encumbered  our  philoso- 
phy. Except  in  a  very  limited  sense,  the  so-called  ele- 
mentary substances  are  now  seen  to  be  as  truly  com- 


viii  PREFACE  TO   REVISED  EDITION. 

pounded  as  any  other  substances,  and  it  is  manifest  that 
their  qualities  must  depend  on  molecular  structure,  or 
on  the  resulting  dynamical  relations,  as  well  as  on  the 
fundamental  attributes  of  the  ultimate  atoms.  There 
is,  therefore,  no  longer  any  reason  for  limiting  the  state- 
ment of  the  great  fundamental  law  of  definite  propor- 
tions to  the  relations  of  elementary  substance,  and 
clearness  of  exposition  is  gained  by  giving  to  this  state- 
ment the  widest  possible  scope. 

But  unquestionably  the  most  important  advance  in 
chemistry  during  the  last  decade  has  resulted  from 
the  study  of  the  thermal  changes  accompanying  chem- 
ical processes,  which  has  proved  that  the  law  of  the 
conservation  of  energy  is  a  directing  principle  in  chem- 
istry as  important  as  it  is  in  physics.  This  study  has 
developed  an  entirely  new  branch  of  our  science  called 
thermo-chemistry ;  and  we  now  confidently  look  for- 
ward to  a  time  in  the  near  future  when  we  shall  be 
able  to  predict  the  order  of  phenomena  in  chemistry  as 
fully  as  we  now  can  in  astronomy. 

So  important  and  fundamental  have  been  the 
changes  required  by  the  recent  progress  that,  in  prepar- 
ing this  book  for  a  new  edition,  the  author  has  found  it 
necessary  to  add  a  great  deal  of  new  material  and  in 
many  places  to  rewrite  the  old,  but  he  has  endeavored 
to  make  the  new  edition,  like  the  first,  a  popular  expo- 
sition of  the  actual  state  of  the  science. 

CAMBRIDGE,  U.  S.  A.,  October  22,  1883. 


PEEFACE. 


THE  lectures  now  published  were  delivered  before 
the  Lowell  Institute,  in  Boston,  in  the  autumn  of  1872. 
They  aimed  to  present  the  modern  theories  of  chem- 
istry to  an  intelligent  but  not  a  professional  audience, 
and  to  give  to  the  philosophy  of  the  science  a  logi- 
cal consistency,  by  resting  it  on  the  Taw  of  Avogadro. 
Since  many  of  the  audience  had  studied  the  elements 
of  chemistry,  as  they  were  formerly  taught  under  the 
dualistic  system,  it  was  also  made  an  object  to  point  out 
the  chief  characteristics  by  which  the  new  chemistry 
differed  from  the  old.  The  limitations  of  a  course  of 
popular  lectures  necessarily  precluded  a  full  presenta- 
tion of  the  subject,  and  only  the  more  prominent  and 
less  technical  features  of  the  new  system  were  discussedr 
In  writing  out  his  notes  for  the  press,  the  author  has 
retained  the  lecture  style,  because  it  is  so  well  adapted 
for  the  popular  exposition  of  scientific  subjects ;  but  he 


x  PREFACE. 

is  painfully  conscious  that  any  description  of  experi- 
ments must  necessarily  fall  far  short  of  giving  that  force 
of  impression  which  the  phenomena  of  Nature  produce 
when  they  speak  for  themselves,  and,  in  weighing  the 
arguments  presented,  he  must  beg  his  readers  to  make 
allowances  for  this  fact. 

CAMBRIDGE,  September  6,  1873. 


ENTKODFCTION. 


IN  most  works  on  chemistry  this  subject  is  defined 
as  the  science  which  treats  of  the  composition  of  bodies ; 
and  it  is  made  the  chief  object  to  present  the  scheme 
of  the  chemical  elements,  and  to  show  that,  by  com- 
bining these  elements,  the  innumerable  products  of 
nature  and  the  arts  may  be  prepared;  and,  although 
the  fundamental  laws  of  the  science  may  be  fully  illus- 
trated, the  discussion  of  these  general  principles  is  made 
a  subordinate  feature  of  the  work. 

In  the  larger  treatises,  which  must  consist  chiefly 
of  descriptions  of  substances  and  processes,  this  method 
of  treatment  is  both  natural  and  necessary.  But  the 
same  plan  is  almost  invariably  adopted  in  the  element- 
ary text-books,  which  are  made  for  the  most  part  com- 
pilations of  facts,  and  differ  from  the  larger  works 
chiefly  in  the  brevity  and  consequent  incompleteness 
of  their  descriptions.  To  the  great  mass  of  learners 
the  study  of  these  text-books  is  uninteresting  and  prof- 
itless ;  for,  before  the  student  is  made  familiar,  through 
long  laboratory  practice,  with  the  materials  and  pro* 
cesses  described,  such  a  book  is  little  more  to  him  than 
a  catalogue  of  names  to  which  he  attaches  no  signifi- 
cance. 


xii  INTRODUCTION. 

While,  however,  the  facts  of  chemistry  have  mul- 
tiplied to  an  extent  that  renders  it  impossible  to 
present  them,  even  briefly,  in  a  volume  of  moderate 
size,  the  general  principles  of  the  science  have  been 
so  developed  that  they  now  form  an  important  body 
of  scientific  truths,  which  may  be  studied  to  advan- 
tage by  themselves,  before  attempting  to  grasp  the 
great  scheme  which  the  composition  of  material  nature 
presents. 

On  this  plan  the  present  work  has  been  written. 
The  aim  has  been  to  develop  the  general  principles  of 
chemistry  in  a  systematic  order,  and  only  so  far  to 
describe  substances  and  processes  as  seemed  necessary 
to  illustrate  these  principles. 

Chemistry  is  defined  as  the  science  which  treats  of 
those  phenomena  of  nature  that  involve  a  change  of 
substance,  and  such  phenomena  are  defined  as  chemical 
processes.  It  is  shown  that  a  chemical  process  always 
consists  in  the  change  of  certain  substances  called  the 
factors  into  certain  other  substances  called  the  products, 
and  that  the  first  object  of  chemical  investigation  is  to 
determine,  in  regard  to  each  chemical  process,  what  are 
the  factors  and  what  are  the  products.  It  is  further 
shown  that  every  chemical  process  obeys  three  funda- 
mental laws :  1.  That  the  sum  of  the  weights  of  the 
products  equals  the  sum  of  the  weights  of  the  factors. 
2.  That  the  relative  weights  of  the  several  factors  and 
products  bear  to  each  other  a  definite  ratio.  3.  That, 
if  the  factors  or  products  are  aeriform,  the  volumes  of 
such  vapors  or  gases  are  very  simply  related.  These 
laws  are  called  respectively — 

The  Law  of  Conservation  of  Mass ; 

The  Law  of  Definite  Proportions ; 

The  Law  of  Gay-Lussac. 


INTRODUCTION.  xiii 

It  will  be  noticed  that  the  fundamental  laws  of 
chemistry,  thus  enunciated,  are  facts  capable  of  simple 
experimental  illustration,  and  involve  no  hypothesis 
whatsoever.  At  this  point,  however,  the  molecular  the- 
ory, by  which  these  laws  are  explained  and  shown  to  be 
related  to  a  system  of  science,  is  introduced.  The  way 
has  already  been  prepared  by  stating  the  general  prin- 
ciples of  the  kinetic  theory  of  gases,  by  which  mole- 
cules are  defined,  and  their  relative  weights  established  ; 
involving  the  well-known  laws  of  Mariotte,  of  Charles, 
and  of  Avogadro. 

It  is  next  made  to  appear  that  the  molecular  weights 
deduced  from  the  kinetic  theory  are  very  simply  re- 
lated to  the  definite  proportions  observed  in  chemical 
processes  ;  and  thus  we  are  led  to  the  chemical  as  dis- 
tinguished from  the  physical  conception  of  the  mole- 
cule, and  it  is  shown  how  greatly  the  coincidence  be- 
tween the  chemical  and  the  physical  results  confirms 
the  molecular  theory.  The  molecule  having  been  fur- 
ther defined  as  the  smallest  mass  in  which  the  qualities 
of  a  substance  inhere,  it  is  made  clear  that  in  all  chemi- 
cal processes  the  action  must  be  referred  to  the  mole- 
cules of  which  the  several  masses  of  the  factors  and 
products  are  aggregates. 

Thus  far  nothing  has  been  said  about  the  compo- 
sition of  matter ;  but  it  is  now  shown  that  the  study 
of  chemical  processes  requires  us  to  admit  that  in 
some  cases  the  material  of  a  product  was  formerly  a 
part  of  the  material  of  a  factor,  while  in  other  cases 
the  material  of  two  or  more  factors  has  united  to  form 
the  material  of  a  single  product.  Hence  arise  neces- 
sarily our  conceptions  of  decomposition  or  composition^ 
of  analysis  or  synthesis  ;  and  we  thus  easily  reach  the 
further  conception  of  a  class  of  substances,  which,  while 


xiy  INTRODUCTION. 

capable  of  synthesis,  are  incapable  of  analysis.  These 
are  the  elementary  substances;  and  although  at  this 
stage  the  complex  processes  of  chemical  analysis  can 
not  be  fully  explained,  yet  the  general  principles  may 
be  made  intelligible,  and  the  method  of  expressing  the 
percentage  composition  of  chemical  compounds  clearly 
stated. 

From  this  stage  in  the  development  of  our  chemical 
philosophy  we  take  the  next  important  step  without 
difficulty.  Since  the  qualities  of  a  substance  inhere  in 
its  molecules,  the  composition  of  the  molecule  must  be 
the  same  as  the  composition  of  the  substance  ;  and  the 
percentage  of  any  element,  found  from  an  analysis  of 
a  mass  of  the  substance,  is  the  percentage  of  that  ele- 
ment in  the  molecule  itself.  These  elementary  parts 
of  the  molecules  are  the  atoms  of  chemistry,  and  we 
thus  reach  not  only  a  conception  of  the  smallest  par- 
ticles into  which  matter  has  been  subdivided,  but  also 
attain  to  a  knowledge  of  the  general  method  by  which 
the  atomic  weights  have  been  established. 

When  the  conceptions  which  modern  chemistry 
connects  with  the  words  molecule  and  atom  have  been 
realized  by  the  student,  the  meaning  of  the  symbolical 
language  of  the  science  is  made  intelligible  with  only 
a  few  words  of  explanation.  The  simple  symbols  stand 
for  the  atoms,  with  their  invariable  relative  weights ; 
molecules  are  represented  by  writing  together  the  sym- 
bols of  the  atoms  of  which  they  consist,  indicating  the 
number  of  atoms  of  each  kind  by  a  subscript  Arabic 
numeral ;  and  these  molecular  formulae  indicate  not 
only  the  molecular  weight,  but  all  such  formulae  also 
represent  equal  gas- volumes.  Lastly,  chemical  processes 
are  represented  by  writing  the  formulae  of  the  mole- 
cules of  the  factors  as  the  first  member,  and  those  of 


INTRODUCTION.  XV 

tlie  products  as  the  second  member,  of  an  equation ; 
indicating  by  numerical  co-efficients  before  each  of  these 
terms  the  number  of  the  molecules  of  each  substance 
involved  in  the  reaction.  After  this  simple  system  of 
symbols  has  been  described,  it  becomes  evident  that  our 
chemical  equations  not  only  accurately  represent  the 
relations  of  a  chemical  process,  but  also  that  they  are 
constant  declarations  of  the  three  great  fundamental 
laws  of  chemistry  already  stated — the  Law  of  Conserva- 
tion of  Mass ;  the  Law  of  Definite  Proportions ;  and 
the  Law  of  Gay-Lussac. 

A  full  command  of  the  symbolical  language  of 
chemistry  is  so  essential  to  every  student  of  the  science 
that  we  next  illustrate  its  use  by  a  number  of  examples 
which  are  so  selected  as  to  prepare  the  way  for  a  further 
development  of  the  subject.  These  examples  include 
the  phenomena  of  combustion  as  a  preparation  for  the 
grand  generalizations  of  therrao-chemistry. 

The  conception  of  the  molecule  as  a  system  of  atoms 
Laving  been  fully  grasped,  the  next  step  is  to  bring  for- 
ward the  evidence  of  molecular  structure,  and  to  illus- 
trate the  doctrine  of  quanti valence,  or  atomicity.  Be- 
ginning with  the  compounds  called  hydrates,  including 
those  important  chemical  agents  the  acids  and  alkalies, 
and  first  showing  that  the  characteristic  qualities  of 
these  bodies  depend  upon  a  common  feature  in  their 
molecular  structure,  known  as  the  hydroxyl  group,  we 
readily  develop  the  subject  so  far  as  to  give  a  general 
idea  of  the  conceptions  of  modern  structural  chemistry, 
and  of  the  striking  results  to  which  it  has  led ;  not  for- 
getting to  point  out  how  far  our  representations  of 
molecular  structure  are  conventional  and  how  far  they 
embody  undoubted  truth. 

Having  recognized  the  very  great  difference  in  the 


XVI  INTRODUCTION. 

stability  of  molecules,  whether  resulting  from  their 
own  structure  or  from  their  association  with  each  other, 
and  seeing  the  manifest  tendency  of  chemical  processes 
to  the  products  of  greatest  stability,  we  next  study  the 
facts  which  prove  that,  while  a  change  from  a  more  to 
a  less  stable  substance  is  attended  with  the  absorption 
of  heat,  and  therefore  requires  the  expenditure  of 
energy,  the  reversion  to  the  stable  condition  is  accom- 
panied by  an  equal  evolution  of  heat,  or  a  correspond- 
ing manifestation  of  energy ;  and  we  discover  that  the 
familiar  phenomena  of  combustion  are  merely  striking 
examples  under  this  general  law.  We  thus  reach  tho 
conception  that  by  conditions  of  structure  the  atoms  of 
molecules  may  be  held  apart  from  those  more  intimate 
associations  into  which  the  atomic  forces  tend  to  bring 
them  ;  and  from  the  analogy  of  the  constructions  of 
man,  in  which  large  masses  of  masonry  are  held  above 
the  surface  of  the  earth  in  opposition  to  the  force  of 
gravitation,  but  fall  in  ruin  when  the  supports  crumble, 
we  gain  a  clearer  idea  of  the  nature  of  chemical  pro- 
cesses. Such  processes  are  now  seen  to  be  in  harmony 
with  the  general  order  of  nature  and  with  the  great 
law  of  conservation  of  energy.  When  the  sun-rays, 
acting  on  the  green  leaves  of  the  plants,  generate  the 
products  of  organic  life,  they  do  work  like  that  of  those 
elder  builders  who  spread  over  many  a  consecrated 
shrine  magnificent  carved  vaultings  secured  in  place 
by  accurately  fitting  and  balancing  the  massive  blocks 
of  stone ;  and  now  when  key-stone  or  buttress  fail,  and 
the  fret-work  comes  tumbling  down,  the  energy  devel- 
oped in  the  fall  furnishes  as  a  similitude  of  the  wonder- 
ful manifestations  of  power  which  accompany  the  fall- 
ing back  of  organic  products  into  the  stable  materials 
from  which  they  originally  sprang. 


INTRODUCTION. 

Finally,  if  the  union  of  atoms  is  attended  with  an 
ever-increasing  evolution  of  heat  as  they  press  together 
into  closer  and  closer  associations,  we  should  naturally 
expect  that  the  effect  of  increasing  temperature  would 
be  to  part  the  atoms ;  and  as  we  study  the  phenomena 
of  disassociation  we  are  led  to  the  latest  conception  of 
chemical  philosophy,  that  of  a  condition  of  disassociated 
atoms  out  of  which  the  material  universe  has  been  devel- 
oped. Such  isolated  atoms  are  for  the  present  at  least 
the  ultimate  elements  of  chemistry,  and  before  reaching 
this  condition  all  qualities  which  distinguish  substances 
disappear  except  only  a  definite  mass  whose  rhythmic 
pulsations  the  spectroscope  may  reveal.  As  out  of  such 
a  primal  chaotic  condition  molecular  structures  were 
evolved,  the  qualities  of  substances  appeared,  and  the 
energy  of  nature  was  awakened.  To  discover  the  laws 
of  this  evolution  so  as  to  follow  its  various  steps  and 
be  able  to  predict  the  results  under  given  conditions,  is 
the  future  work  of  chemistry. 

The  plan  of  developing  the  principles  of  chemistry 
sketched  above  is  suitable  not  only  for  a  popular  pres- 
entation of  the  subject  like  that  in  this  volume,  but 
also  for  a  course  of  laboratory  teaching.  In  such  a 
course  every  point  in  the  reasoning  should  be  fully 
enforced  by  experiments,  which  should  be  so  devised 
that  the  student  will  be  led  to  the  result  inductively, 
and  at  the  same  time  will  understand  the  limitations 
within  which  the  principle  illustrated  holds  true.  On 
this  plan  his  interest  can  be  sustained  to  the  end,  which 
is  hardly  possible  in  following  through  the  weary  cata- 
logue of  elementary  substances,  involving  a  repetition 
of  details  as  profitless  to  the  general  student  as  it  is 
tedious  and  uninteresting. 

CAMBRIDGE,  April  22,  188JJ,. 


CONTENTS. 


LECTURE  PAGE 

I.   MOLECULES  AND  AYOGADRO'S  LAW       ....  1 

II.   THE  MOLECULAR  CONDITION  OF  THE  THREE  STATES  OF  MAT- 
TER— THE  GAS,  THE  LIQUID,  AND  THE  SOLID        .        .  29 

III.  How  MOLECULES  ARE  WEIGHED   .        .        .        .        .        .65 

IV.  LAW  OF  CONSERVATION  OF  MASS — LAW  OF  DEFINITE  PRO- 

PORTIONS, AND  LAW  OF  GAY-LUSSAC    .        .        .        .  86 
V.   CHEMICAL   COMPOSITION  —  ANALYSIS   AND   SYNTHESIS  —  THE 

ATOMIC  THEORY 98 

VI.   ELEMENTARY  SUBSTANCES  AND  COMBINING  PROPORTIONS  .  119 
VII.   ATOMIC  WEIGHTS  AND  CHEMICAL  SYMBOLS  .        .        .        .138 

VIII.   CHEMICAL  REACTIONS 165 

IX.   CHEMICAL  CHANGES  CLASSIFIED 191 

X.   THE  THEORY  OF  COMBUSTION 216 

XI.   GUNPOWDER  AND  NITRO-GLYCERINE 237 

XII.   METATHESIS  AND  QUANTIYALENCE — ALKALIES  AND  ACIDS.  257 

XIII.  ELECTRO-CHEMICAL  THEORY^ 290 

XIV.  ISOMERISM,    AND   THE    SYNTHESIS    OF   ORGANIC    COMPOUNDS.  321 

XV.   THERMO-CHEMISTRY 355 


THE    NEW    CHEMISTRY. 


LECTUEE  I. 

MOLECULES   AND  AVOGADRo's   LAW. 

IN  every  physical  science  we  have  carefully  to  dis- 
tinguish between  the  facts  which  form  its  subject-mat- 
ter and  the  theories  by  which  we  attempt  to  explain 
these  facts,  and  group  them  in  our  scientific  systems. 
The  first  alone  can  be  regarded  as  absolute  knowledge, 
and  such  knowledge  is  immutable,  except  in  so  far  as 
subsequent  observation  may  correct  previous  error. 
The  last  are,  at  best,  only  guesses  at  truth,  and,  even 
in  their  highest  development,  are  subject  to  limitations, 
and  liable  to  change. 

But  this  distinction,  so  obvious  when  stated,  is  often 
overlooked  in  our  scientific  text-books,  and  not  without 
reason,  for  it  is  the  sole  aim  of  these  elementary 
treatises  to  teach  the  present  state  of  knowledge,  and 
they  might  fail  in  their  object  if  they  attempted,  by  a 
too  critical  analysis,  to  separate  the  phenomena  from 
the  systems  by  which  alone  the  facts  of  Nature  are 
correlated  and  rendered  intelligible. 

When,  however,  we  come  to  study  the  history  of 
science,  the  distinction  between  fact  and  theory  ob- 
trudes itself  at  once  upon  our  attention.  We  see 
that,  while  the  prominent  facts  of  science  have  re- 


2  MOLECULES  AND  AVOGADRO'S  LAW. 

mained  the  same,  its  history  has  been  marked  by  very 
frequent  revolutions  in  its  theories  or  systems.  The 
courses  of  the  planets  have  not  changed  since  they 
were  watched  by  the  Chaldean  astronomers,  three  thou- 
sand years  ago  ;  but  how  differently  have  their  motions 
been  explained — first  by  Hipparchus  and  Ptolemy, 
then  by  Copernicus  and  Kepler,  and  lastly  by  Newton 
and  Laplace ! — and,  however  great  our  faith  in  the  law 
of  universal  gravitation,  it  is  difficult  to  believe  that 
even  this  grand  generalization  is  the  final  result  of 
astronomical  science. 

Let  me  not,  however,  be  understood  to  imply  a  be- 
lief that  man  cannot  attain  to  any  absolute  scientific 
truth ;  for  I  believe  that  he  can,  and  I  feel  that  every 
great  generalization  brings  him  a  step  nearer  to  the 
promised  goal.  Moreover,  I  sympathize  with  that 
beautiful  idea  of  Oersted,  which  he  expressed  in  the 
now  familiar  phrase,  "  The  laws  of  Nature  are  the 
thoughts  of  God;"  but,  then,  I  also  know  that  our 
knowledge  of  these  laws  is  as  yet  very  imperfect,  and 
that  our  human  systems  must  be  at  the  best  but  very 
partial  expressions  of  the  truth.  Still,  it  is  a  fact,  wor- 
thy of  our  profound  attention,  that  in  each  of  the  physi- 
cal sciences,  as  in  astronomy,  the  successive  great  gen- 
eralizations which  have  marked  its  progress  have  in- 
cluded and  expanded  rather  than  superseded  those  which 
went  before  them.  Through  the  great  revolutions  which 
have  taken  place  in  the  forms  of  thought,  the  elements 
of  truth  in  the  successive  systems  have  been  preserved, 
while  the  error  has  been  as  constantly  eliminated  ; 
and  so,  as  I  believe,  it  always  will  be,  until  the  last 
generalization  of  all  brings  us  into  the  presence  of  that 
law  which  is  indeed  the  thought  of  God. 

There  is  also  another  fact,  which  has  an  important 


ANTICIPATION  IN  SCIENCE.  3 

bearing  on  the  subject  we  are  considering.  Almost 
all  the  great  generalizations  of  science  have  been  more 
or  less  fully  anticipated,  at  least  in  so  far  that  the  gen- 
eral truth  which  they  involve  has  been  previously 
conceived.  The  Copernican  theory  was  taught,  sub- 
stantially, by  the  disciples  of  Pythagoras.  The  law 
of  gravitation  was  suggested,  both  by  Hooke  and 
Cassini,  several  years  before  Newton  published  his 
"Principia;"  and  the  same  general  fact  has  been 
recently  very  markedly  illustrated  in  the  discovery  of 
the  methods  of  spectrum  analysis,  every  principle  of 
which  had  been  previously  announced.  The  history 
of  science  shows  that  the  age  must  be  prepared  before 
really  new  scientific  truths  can  take  root  and  grow. 
The  barren  premonitions  of  science  have  been  barren 
because  these  seeds  of  truth  fell  upon  unfruitful  soil ; 
and>  as  soon  as  the  fulness  of  the  time  was  come,  the 
seed  has  taken  root  and  the  fruit  has  ripened.  No 
onS  can  doubt,  for  example,  that  the  law  of  gravitation 
would  have  been  discovered  before  the  close  of  the 
seventeenth  century  if  Newton  had  not  lived ;  and  it  is 
equally  true  that,  had  Newton  lived  before  Galileo  and 
Kepler,  he  never  could  have  mastered  the  difficult 
problems  it  was  his  privilege  to  solve.  "We  justly  honor 
with  the  greatest  veneration  the  true  men  who,  having 
been  called  to  occupy  these  distinguished  places  in  the 
history  of  science,  have  been  equal  to  their  position, 
and  have  acquitted  themselves  so  nobly  before  the 
world  ;  but  every  student  is  surprised  to  find  how  very 
little  is  the  share  of  new  truth  which  even  the  greatest 
genius  has  added  to  the  previous  stock.  Science  is  a 
growth  of  time,  and,  though  man's  cultivation  of  the 
field  is  an  essential  condition  of  that  growth,  the  de- 
velopment steadily  progresses,  independently  of  any  in- 


4  MOLECULES  AND  AVOGADRO'S   LAW. 

dividual  investigator,  however  great  his  mental  power. 
The  greatest  philosophical  generalizations,  if  prema- 
ture, will  fall  on  barren  soil,  and,  when  the  age  is 
ripe,  they  are  never  long  delayed.  The  very  discovery 
of  law  is  regulated  by  law,  or,  as  we  rather  believe,  is 
directed  by  Providence ;  but,  however  we  may  prefer 
to  represent  the  facts,  this  natural  growth  of  knowl- 
edge gives  us  the  strongest  assurance  that  the  growth 
is  sound  and  the  progress  real.  Although  the  foun- 
dations of  science  have  been  laid  in  such  obscurity,  its 
students  have  worked  under  the  direction  of  the  same 
guiding  power  which  rules  over  the  whole  of  Nature, 
and  it  cannot  be  that  the  structure  they  have  reared 
with  so  much  care  is  nothing  but  the  phantom  of  a 
dream.  Still  it  is  true  that,  beyond  the  limits  of  direct 
observation,  our  science  is  not  infallible,  and  our  theo- 
ries and  systems,  although  they  may  all  contain  a  ker- 
nel of  truth,  undergo  frequent  changes,  and  are  often 
revolutionized. 

Through  such  a  revolution  the  theory  of  chemistry 
has  recently  passed,  and  the  system  which  is  now  uni- 
versally accepted  by  the  principal  students  of  the  sci- 
ence is  greatly  different  from  that  which  has  been 
taught  in  our  schools  and  colleges  until  within  a  few 
years.  I  have,  therefore,  felt  that  the  best  service  I 
could  render  in  this  course  of  lectures  would  be  to  ex- 
plain, as  clearly  as  I  am  able,  the  principles  on  which 
the  new  philosophy  is  based,  and  to  show  in  what  it 
differs  from  the  old.  I  have  felt  that  there  were  many 
who,  having  studied  what  we  must  now  call  the  old 
chemistry,  would  be  glad  to  bridge  over  the  gulf  which 
separates  it  from  the  new,  and  to  become  acquainted 
with  the  methods  by  which  we  now  seek  to  group  to- 
gether  and  explain  the  old  facts. 


STARTING-POINT  OF  THE  NEW  CHEMISTRY.  5 

Those  who  studied  the  science  of  chemistry  twenty 
years  ago,  as  it  was  taught,  for  example,  in  tne  works 
of  the  late  Dr.  Turner,  were  greatly  impressed  witii 
the  simplicity  of  the  system  and  the  beauty  of  its  no- 
menclature. Until  recently  the  study  of  the  new  chemis- 
try has  been  far  less  inviting ;  since  the  science  has  been 
passing  through  a  process  of  reconstruction,  and  dis- 
played the  imperfections  of  any  half-built  edifice  ;  but 
it  has  now  reached  a  condition  in  which  it  can  be  pre- 
sented with  the  unity  of  a  philosophical  system.  Our 
starting-point  in  the  exposition  of  the  modern  chemis- 
try must  be  the  great  generalization  wThich  is  now 
known  as  the  law  of  Avogadro,  or  Ampere.  This 
law  was  first  stated  by  Amedeo  Avogadro,  an  Italian 
physicist,  in  Itll,  and  was  reproduced  by  Ampere,  a 
French  physicist,  in  1814.  But,  although  attained 
thus  early  in  the  history  of  our  science,  this  grand 
conception  remained  barren  for  nearly  half  a  century. 
Now,  however,  it  holds  the  same  place  in  chemistry 
that  the  law  of  gravitation  does  in  astronomy,  though, 
unlike  the  latter,  it  was  announced  half  a  century  be- 
fore the  science  was  sufficiently  mature  to  accept  it. 
The  law  of  Avogadro  may  be  enunciated  thus  : 

EQUAL  VOLUMES  OF  ALL  SUBSTANCES,  WHEN  IN  THE 

STATE  OF  GAS,  AND   UNDER   LIKE   CONDITIONS,  CONTAIN  THE 

SAME  NUMBER  OF  MOLECULES  (Avogadro,  1811 — Ampere, 
1814). 

The  enunciation  of  this  law  is  very  simple,  but,  be- 
fore we  can  comprehend  its  meaning,  we  must  under- 
stand what  is  meant  by  the  term  MOLECULE.  This 
word  is  the  one  selected  by  Avogadro  in  the  enuncia- 
tion of  his  law.  It  is  obviously  of  Latin  origin,  and 
means  simply  a  little  mass  of  matter.  Ampere  used  in 


6  MOLECULES  AND  AVOGADRO'S  LAW. 

its  place  the  word  particle,  in  precisely  the  same  sense. 
Both  words  signify  the  smallest  mass  into  which  any 
substance  is  capable  of  being  subdivided  by  physical 
processes  ;  that  is,  by  processes  which  do  not  change  its 
chemical  nature.  In  many  of  our  text-books  it  is  defined 
as  the  smallest  mass  of  any  substance  which  can  exist 
by  itself,  but  both  definitions  are  in  essence  the  same. 

As  this  is  a  very  important  point,  it  must  be  fully 
illustrated.  In  the  first  place,  we  recognize  in  Nature 
a  great  variety  of  different  substances.  Indeed,  on 
this  fact  the  whole  science  of  chemistry  rests ;  for,  if 
Nature  were  made  out  of  a  single  substance,  there 
could  be  no  chemistry,  even  if  there  could  be  intel- 
ligences to  study  science  at  all.  Chemistry  deals 
exclusively  with  the  relations  of  different  substances. 
Now,  these  substances  present  themselves  to  us  under 
three  conditions  :  those  of  the  solid,  the  liquid,  and  the 
gas.  Heat  tends  to  decompose  all  compound  bodies ; 
but  although,  for  this  reason,  there  are  numerous  sub- 
stances that  have  never  been  melted,  and  more  which 
have  never  been  volatilized,  yet  very  many  substances 
can  be  made  to  assume  all  the  three  conditions  of  matter 
named  above.  Thus,  as  every  one  knows,  water  can  most 
readily  be  changed  both  into  solid  ice  and  into  aeriform 
steam.  Let  me  begin  with  this  most  familiar  of  all  sub- 
stances to  illustrate  what  I  mean  by  the  word  molecule. 

When,  by  boiling  under  the  atmospheric  pressure, 
water  changes  into  steam,  it  expands  1,800  times  ;  or, 
in  other  words,  one  cubic  inch  of  water  yields  one 
cubic  foot  of  steam,  nearly.  Now,  two  suppositions 
are  possible  as  modes  of  explaining  this  change. 

The  first  is,  that,  in  expanding,  the  material  of  the 
water  becomes  diffused  throughout  the  cubic  foot,  so 
as  to  fill  the  space  completely  with  the  substance  we 


PARTICLES   SEPARATED   IN   STEAM. 


FIG.  1. 

call  water,  the  resulting  mass  of  steam  being  absolutely 
homogeneous,  so  that  there  is  no  space  within  the  cubic 
foot,  however  minute,  which  does  not  contain  its  prop- 
er proportion  of  water. 

The  second  is,  that  the  cubic  inch  of  water  consists 
of  a  certain  number  of  definite  particles,  which,  in  the 
process  of  boiling,  are  not  subdivided,  so  that  the  cubic 
foot  of  steam  contains  the  same  number  of  the  same 
particles  as  the  cubic  inch  of  water,  the  conversion  of 
the  one  into  the  other  depending  simply  on  the  action 
of  heat  in  separating  these  particles  to  a  greater  dis- 
tance. Hence  the  steam  is  not  absolutely  homogene- 
ous ;  for,  if  we  consider  spaces  sufficiently  minute,  we 
can  distinguish  between  such  as  contain  a  particle  of 
water  and  those  which  lie  between  the  particles.  Now, 
the  small  masses  of  water,  whose  isolation  we  here  as- 
sume, are  what  Avogadro  calls  molecules,  and,  follow- 


MOLECULES  AND  AVOGADKO'S  LAW. 


FIG.  2. 

ing  his  authority,  we  shall  designate  them  hereafter  ex- 
clusively by  this  word. 

The  rude  diagrams  before  you  will  help  me  to  make 
clear  the  difference  between  the  two  suppositions  I 
have  made.  In  the  first  (Fig.  1),  we  assume  that  the 
material  of  this  cubic  inch  is  uniformly  expanded 
through  the  cubic  foot.  In  the  other  (Fig.  2),  we  have 
in  both  volumes  a  definite  number  of  molecules,  the 
only  difference  being  that  these  dots,  which  we  have 
used  to  represent  the  molecules,  are  more  widely  separa- 
ted in  the  one  case  than  in  the  other.  Now,  which  of 
these  suppositions  is  the  more  probable  ?  Let  us  sub- 
mit the  question  to  the  test  of  experiment. 

We  have  here  a  glass  globe,  provided  with  the  nec- 
essary mountings — a  stop-cock,  a  pressure-gauge,  and  a 
thermometer — and  which  we  will  assume  has  a  capacity 
of  one  cubic  foot.  Into  this  globe  we  will  first  pour  one 


INTERSPACES   IN   VAPORS.  9 

cubic  inch  of  water,  and,  in  order  to  reduce  the  condi- 
tions to  the  simplest  possible,  we  will  connect  the 
globe  with  our  air-pump,  and  exhaust  the  air,  al- 
though, as  it  will  soon  appear,  this  is  not  necessary  for 
the  success  of  our  experiment.  Exposing,  next,  the 
globe  to  the  temperature  of  boiling  water,  all  the 
liquid  will  evaporate,  and  we  shall  have  our  vessel 
filled  with  ordinary  steam.  If,  now,  that  cubic  foot  of 
space  is  really  packed  close  writh  the  material  we  call 
wrater — if  there  is  no  break  in  the  continuity  of  the 
aqueous  mass — we  should  expect  that  the  vapor  would 
fill  the  space,  to  the  exclusion  of  every  thing  else,  or, 
at  least,  would  fill  it  with  a  certain  degree  of  energy 
which  must  be  overcome  before  any  other  vapor  could 
be  forced  in.  Now,  what  is  the  case  2  The  stop-cock 
of  the  globe  is  so  arranged  that  we  can  introduce  into 
it  an  additional  quantity  of  any  liquid  on  which  we 
desire  to  experiment,  without  otherwise  opening  the 
vessel.  If,  then,  by  this  means,  we  add  more  water,  the 
additional  quantity  thus  added  will  not  evaporate,  pro- 
vided that  the  temperature  remains  at  the  boiling-point. 
Let  us  next,  however,  add  a  quantity  of  alcohol,  and 
what  do  we  find  ?  Why,  not  only  that  this  immedi- 
ately evaporates,  but  wre  find  that  just  as  much  alcohol- 
vapor  will  form  as  if  no  steam  wrere  present.  The 
presence  of  the  'Steam  does  not  interfere  in  the  least 
degree  with  the  expansion  of  liquid  alcohol  into  alco- 
hol-vapor. The  only  difference  which  we  observe  is, 
that  the  alcohol  expands  more  slowly  into  the  aque- 
ous vapor  than  it  would  into  a  vacuum.  If,  now  that 
the  globe  *is  filled  with  aqueous  vapor  and  alcohol- 
vapor  at  one  and  the  same  time,  each  acting,  in  all  re- 
spects, as  if  it  occupied  the  space  alone,  we  add  a  quan- 
tity of  ether,  we  shall  have  the  same  phenomena  re- 
3 


10  MOLECULES  AND   AVOGADRO'S   LAW. 

peated.  The  ether  will  expand  and  fill  the  space 
with  its  vapor,  and  the  globe  will  hold  just  as  much 
ether- vapor  as  if  neither  of  the  other  two  were  present ; 
and  so  we  might  go  on,  as  far  as  we  know,  indefinitely. 
There  is  not  here  a  chemical  union  between  the  sev- 
eral vapors,  and  we  cannot  in  any  sense  regard  the 
space  as  filled  with  a  compound  of  the  three.  It  con- 
tains all  three  at  the  same  time,  each  acting  as  if  it 
were  the  sole  occupant  of  the  space ;  and  that  this  is 
the  real  condition  of  things  we  have  the  most  unques- 
tionable evidence, 

You  know,  for  example,  that  a  vapor  or  gas  exerts 
a  certain  very  considerable  pressure  against  the  walls 
of  the  containing  vessel.  Now,  each  of  these  vapors 
exerts  its  own  pressure,  and  just  the  same  pressure  as 
if  it  occupied  the  space  alone,  so  that  the  total  pressure 
is  exactly  the  sum  of  the  three  partial  pressures. 

Evidently,  then,  no  vapor  completely  fills  the  space 
which  it  occupies,  although  equally  distributed  through 
it ;  and  we  can  give  no  satisfactory  explanation  of  the 
phenomena  of  evaporation  except  on  the  assumption 
that  each  substance  is  an  aggregate  of  particles,  or  units, 
which,  by  the  action  of  heat,  become  widely  separated 
from  each  other,  leaving  very  large  intermolecular 
spaces,  within  which  the  particles  of  an  almost  indefi- 
nite number  of  other  vapors  may  find  place.  Pass 
now  to  another  class  of  facts,  illustrating  the  same  point. 

The  three  liquids,  water,  alcohol,  and  ether,  are  ex- 
panded by  heat  like  other  forms  of  matter,  but  there  is 
a  striking  circumstance  connected  with  these  phenom- 
ena, to  which  I  wish  to  direct  your  observation.  I  have, 
therefore,  filled  three  perfectly  similar  thermometer- 
bulb  tubes,  eadi  with  one  of  those  liquids.  The  tubes 
are  mounted  in  a  glass  cell  standing  before  the  con- 


UNEQUAL  EXPANSION   IN  LIQUIDS.  11 

denser  of  a  magic  lantern,  and  you  see  their  images 
projected  on  the  screen.  You  also  notice  that  the 
liquids  (which  have  been  colored  to  make  them  visible) 
all  stand  at  the  same  height ;  and,  "since  both  the 
bulbs  and  the  tubes  are  of  the  same  dimensions,  the 
relative  change  in  volume  of  the  inclosed  liquids  will 
be  indicated  by  the  rise  or  fall  of  the  liquid  columns  in 
the  tubes.  We  will  now  fill  the  cell  with  warm  water, 
and  notice  that,  as  soon  as  the  heat  begins  to  penetrate 
the  liquids,  the  three  columns  begin  to  rise,  indicating 
an  increase  of  volume ;  but  notice  how  unequal  is  the 
expansion.  The  ether  in  the  right-hand  tube  expands 
more  than  the  alcohol  in  the  centre,  and  that  again  far 
more  than  the  water  on  the  left.  What  is  true  of 
these  three  liquids  is  true  in  general  of  all  liquids. 
Each  has  its  own  rate  of  expansion,  and  the  amount  in 
any  case  does  not  appear  to  depend  on  any  peculiar 
physical  state  or  condition  of  the  liquid,  but  is  con- 
nected with  the  nature  of  the  substance,  although,  in 
what  way,  we  are  as  yet  wholly  ignorant. 

But  you  may  ask:  What  is  there  remarkable  in 
this  ?  Why  should  we  not  expect  that  the  rate  of  ex- 
pansion would  differ  with  different  substances  ?  Cer- 
tainly, there  is  no  reason  to  be  surprised  at  such  a  fact. 
But,  then,  the  remarkable  circumstance  connected  with 
this  class  of  phenomena  has  yet  to  be  stated. 

Raise  the  temperature  of  these  liquids  to  a  point  a 
little  above  that  of  boiling  water,  and  we  shall  convert 
all  three  substances  into  vapor.  We  thus  obtain  three 
gases,  and,  on  heating  these  aeriform  bodies  to  a  still 
higher  temperature,  we  shall  find  that,  in  this  new  con- 
dition, they  expand  far  more  rapidly  than  in  the  liquid 
state.  But  we  shall  also  find  that  the  influence  of  the 
nature  of  the  substance  on  the  phenomenon  has  wholly 


12  MOLECULES   AND   AVOGADRO'S   LAW. 

disappeared,  and  that,  in  the  aeriform  condition,  these 
substances,  and  in  general  all  substances,  expand  at  the 
same  rate  under  like  conditions. 

Why,  now,  this  difference  between  the  two  states 
of  matter  ?  If  the  material  fills  space  as  completely  in 
the  aeriform  as  it  does  in  the  liquid  condition,  then  we 
cannot  conceive  why  the  nature  of  the  substance  should 
not  have  the  same  influence  on  the  phenomena  of  ex- 
pansion in  both  cases.  If,  however,  matter  is  an  ag- 
gregate of  definite  small  masses  or  molecules,  which, 
while  comparatively  close  together  in  the  liquid  state, 
become  widely  separated  when  the  liquids  are  con- 
verted into  vapor,  then  it  is  obvious  that  the  action  of 
the  particles  on  each  other,  which  might  be  consider- 
able in  the  first  state,  would  become  less  and  less  as 
the  molecules  were  separated,  until  at  last  it  was  inap- 
preciable ;  and  if,  farther,  as  Avogadro's  law  assumes, 
the  number  of  these  particles  in  u  given  space  is  the 
same  for  all  gases  under  the  same  conditions,  then  it  is 
equally  obvious  that,  there  being  no  action  between 
the  particles,  all  vapors  may  be  regarded  as  aggregates 
of  the  same  number  of  isolated  particles  similarly 
placed,  and  we  should  expect  that  the  action  of  heat 
on  such  similar  masses  would  be  the  same. 

Thus  these  phenomena  of  heat  almost  force  upon 
us  the  conviction  that  the  various  forms  of  matter  we 
see  around  us  do  not  completely  fill  the  spaces  which 
they  appear  to  occupy,  but  consist  of  isolated  particles 
separated  by  comparatively  wide  intervals.  There  are 
many  other  facts  which  might  be  cited  in  support  of 
the  same  conclusion  :  and  among  these  two,  which  are 
more  especially  worthy  of  your  attention,  because  they 
aid  us  in  forming  some  conception  of  the  size  of  the 
molecules  themselves. 


INTERSTICES   IN   SOLIDS.  13 

If  this  mass  of  glass  is  perfectly  homogeneous — if 
the  vitreous  substance  completely  fills  its  allotted  space, 
and  there  is  no  break  whatever  in  the  continuity  of 
the  material — then  you  would  expect  that  its  physical 
relations  would  not  depend  at  all  on  the  size  of  the 
surface  affected.  Suppose  you  wished  to  penetrate  it 
with  a  fine  wire.  The  point  of  this  wire,  however 
small,  would  not  detect  any  difference  at  different 
points  of  the  surface.  Assume,  however,  that  it  con- 
sists of  masses  separated  by  spaces,  like,  for  example, 
this  sheet  of  wire  netting.  Then,  although  the  surface 
would  seem  perfectly  homogeneous  to  a  bar  large 
enough  to  cover  a  number  of  meshes,  it  would  not  be 
found  to  be  by  any  means  homogeneous  to  a  wire 
which  was  small  enough  to  penetrate  the  meshes.  If, 
now,  there  are  similar  interstices  in  this  mass  of  glass, 
we  should  expect  that,  if  our  wire  were  small  enough 
(that  is.  of  dimensions  corresponding  to  the  interstices), 
it  would  detect  differences  in  the  resistance  at  different 
points  of  this  glass  surface. 

Make,  now,  a  further  supposition.  Assume  that 
we  have  a  number  of  these  wires  of  different  sizes,  the 
largest  being  twice  as  stout  as  the  smallest.  It  is  ob- 
vious that,  if  the  interstices  we  have  assumed  were,  say, 
several  thousand  times  larger  than  the  largest  wire,  all 
the  wires  would  meet  with  essentially  the  same  oppo- 
sition when  thrust  at  the  glass.  If,  however,  the  inter- 
stices were  only  four  or  five  times  larger  than  the  wires, 
then  the  larger  would  encounter  much  greater  resist- 
ance from  the  edges  of  the  meshes  than  the  smaller. 

It  is  unnecessary  to  say  that  no  physical  point  can 
detect  an  inequality  in  the  surface  of  a  plate  of  glass, 
but  we  have,  in  what  we  call  a  beam  of  light,  an  agent 
which,  in  passing  through  its  mass,  does  discover  differ- 
ences  of  the  kind  we  have  attempted  to  describe.  Now,  it 


14  MOLECULES  AND   AVOGADRO'S  LAW. 

is  perfectly  true  that  we  liave  no  absolute  knowledge 
of  the  nature  of  a  beam  of  light.  We  have  a  very 
plausible  theory  that  the  phenomena  of  light  are  the 
effects  of  waves  transmitted  through  a  highly-elastic 
medium  we  call  ether,  and  that,  in  the  case  of  our  plate 
of  glass,  the  motion  is  transmitted  through  the  ether, 
which  tills  the  interstices  between  the  molecules  of  this 
transparent  solid ;  but  we  have  no  right  to  assume  this 
theory  in  our  present  discussion. 

Indeed,  I  cannot  agree  with  those  who  regard  the 
wave-theory  of  light  as  an  established  principle  of 
science.  That  it  is  a  theory  of  the  very  highest  value 
I  freely  admit,  and  that  it  has  been  able  to  predict  the 
phases  of  unknown  phenomena,  which  experiment  has 
subsequently  brought  to  light,  is  a  well-known  fact. 
All  this  is  true ;  but  then,  on  the  other  side,  the  theory 
requires  a  combination  of  qualities  in  the  ether  of  space, 
which  I  find  it  difficult  to  believe  are  actually  realized. 
For  instance,  the  rapidity  with  which  wave-motion  is 
transmitted  depends,  other  things  being  equal,  on  the 
elasticity  of  the  medium.  Assuming  that  two  media 
have  the  same  density,  their  elasticities  are  proportional 
to  the  squares  of  the  velocities  with  which  a  wave  trav- 
els. The  velocity  of  the  sound-wave  in  air  is  about 
1,100  feet  a  second  or  -^  of  a  mile,  that  of  the  light- 
wave about  192,000  miles  a  second,  or  about  one  million 
times  greater ;  and,  if  we  take  into  account  certain 
causes,  which,  though  they  tend  to  increase  the  velocity 
of  sound,  can  have  no  effect  on  the  luminiferous  ether, 
the  difference  would  be  even  greater  than  this. 

Now,  were  the  density  of  the  ether  as  great  as  that 
of  the  atmosphere  (say  -J  of  a  grain  to  the  cubic  inch), 
its  elasticity  or  power  of  resisting  pressure  would  be  a 
million  square,  or  a  million  million  times  that  of  the 


DIFFICULTIES  WITH  THE  ETHEK.  15 

atmosphere.  But,  as  you  well  know,  the  atmosphere 
can  resist  a  pressure  of  about  fifteen  pounds  to  the  square 
inch ;  hence  the  ether,  when  equally  dense,  would  re- 
sist a  pressure  of  fifteen  million  million  pounds  to  the 
square  inch,  or,  making  the  correction  referred  to 
above,  seventeen  million  million  pounds  to  the  square 
inch.  Of  course,  such  numbers  convey  no  impression, 
except  that  of  vast  magnitude ;  and  you  will  obtain  a 
clearer  idea  of  the  power  when  I  tell  you  that  this 
pressure  is  about  the  weight  of  a  cubic  mile  of  granite 
rock.  Here  is  a  glass  cylinder  filled  with  air,  and  here 
a  piston  which  just  fits  it.  The  area  of  the  piston  is 
about  a  square  inch — we  wrill  assume  that  it  is  exactly 
that.  If  we  put  a  weight  of  fifteen  pounds  on  the  top 
of  the  piston,  it  will  descend  just  half-way  in  the  tube, 
and  the  air  will  be  condensed  to  twice  its  normal 
density.  Now,  if  we  had  a  cylinder  and  piston,  ether- 
tight  as  this  is  air-tight,  and  of  sufficient  strength,  and, 
if  we  put  on  top  of  it  a  cubic  mile  of  granite  rock,  it 
wrould  only  condense  the  ether  to  about  the  same  den- 
sity as  that  of  the  atmosphere  at  the  surface  of  the 
earth.  Of  course,  the  supposition  is  an  absurdity,  for 
it  is  assumed  that  the  ether  pervades  the  densest  solids 
as  readily  as  water  does  a  sponge,  and  could  not,  there- 
fore, be  confined ;  but  the  illustration  will  give  you  an 
idea  of  the  nature  of  the  medium  which  the  undulatory 
theory  assumes.  It  is  a  medium  so  thin  that  the  earth, 
moving  in  its  orbit  1,100  miles  a  minute,  suffers  no  per- 
ceptible retardation,  and  yet  endowed  with  an  elasticity 
in  proportion  to  its  density  a  million  million  times 
greater  than  air. 

Whether,  however,  there  are  such  things  as  waves 
of  ether  or  not,  there  is  something  concerned  in  the 
phenomena  of  light  which  has  definite  dimensions,  that 


16 


MOLECULES  AND  AVOGADRO'S  LAW. 


have  been  measured  with  as  much  accuracy  as  the  di- 
mensions of  astronomy,  although  they  are  at  the  oppo- 
site extreme  of  the  scale  of  magnitude.  We  represent 
these  dimensions  to  our  imagination  as  wave-lengths, 
that  is,  as  the  distances  from  crest  to  crest  of  our  as- 
sumed ether-waves,  and  we  shall  find  it  difficult  to 
think  clearly  upon  the  subject  without  the  aid  of  this 
wave-theory,  and  every  student  of  physics  will  bear  me 
out  in  the  statement  that,  though  our  theory  may  be  a 
phantom  of  our  scientific  dreaming,  these  magnitudes 
must  be  the  dimensions  of  something.  Here  they  are : 

Dimensions  of  Light-waves. 


COLORS. 

Number  of  waves  in 
one  inch. 

Number  of  oscillations 
in  one  second. 

Red               

89,000 

477,000,000,000,000 

Orange  

42,000 

506,000,000,000,000 

Yellow  

44,000 

535,000,000,000,000 

Green            

47,000 

577,000,000,000,000 

Blue.                          

51,000 

622,000,000,000,000 

Indigo  

54,000 

658,000,000,000,000 

Violet 

57,000 

699,000,000,000,000 

You  know  that  the  sensation  we  call  white  light  is 
a  very  complex  phenomenon,  and  is  produced  by  rays 
of  all  colors  acting  simultaneously  on  the  eye.  A  very 
pretty  experiment  will  illustrate  this  point.  I  have 
projected  on  the  screen  the  image  of  a  circular  disk 
made  of  sectors  of  gelatine-paper,  variously  colored. 
By  means  of  a  very  simple  apparatus,  I  can  revolve  the 
disk,  and  thus  cause  the  several  colors  to  succeed  each 
other  at  the  same  point  with  great  rapidity,  and  you 
notice  that  the  confused  effect  of  the  different  colors 
produces  the  impression  you  call  white,  or,  at  least, 
nearly  that. 

The  sunbeam  produces  the  same  impression,  be- 


MAGNITUDES  OF  ETHER-WAVES.  17 

cause  it  contains  all  these  colored  rays ;  and,  if  we  pass 
it  through  a  prism,  the  several  rays,  being  bent  un- 
equally by  the  glass,  diverge  on  emerging,  so  that,  if 
we  receive  the  beam  thus  divided  on  a  screen  placed  at 
a  sufficient  distance,  we  obtain  that  magnificent  band 
of  blending  hues  we  call  the  solar  spectrum. 

To  each  of  the  colored  rays  which  fall  along  the 
line  of  the  spectrum  corresponds  a  definite  wave- 
length. In  the  diagram,  we  have  given  the  wave- 
lengths, corresponding  to  only  a  few  selected  points, 
one  in  each  color,  and  marked  in  the  solar  spectrum 
itself  by  certain  remarkable  dark  lines  by  wrhich  it 
is  crossed.  These  values  always  create  a  smile  with 
a  popular  audience,  which  makes  it  evident  that,  by 
those  unfamiliar  with  the  subject,  they  are  looked  upon 
as  unreal  if  not  absurd.  But  this  is  a  prejudice.  In 
our  universe  the  very  small  is  as  real  as  the  very 
great ;  and  if  science  in  astronomy  can  measure  dis- 
tances so  great  that  this  same  swift  messenger,  light, 
traveling  192,000  miles  a  second,  requires  years  to 
cross  them,  we  need  not  be  surprised  that,  at  the  other 
end  of  the  scale,  it  can  measure  magnitudes  like  these. 

Let  not,  then,  these  numbers  impair  your  confidence 
in  our  results ;  but  remember  that  the  microscope  re- 
veals a  universe  with  dimensions  of  the  same  order  of 
magnitude.  Moreover,  the  magnitudes  with  which  we 
are  here  dealing  are  not  beyond  the  limits  of  mechani- 
cal skill.  It  is  possible  to  rule  lines  on  a  plate  of  glass 
so  close  together  that  the  bands  of  fine  lines  thus  ob- 
tained cannot  be  resolved  even  by  the  most  powerful 
microscopes ;  and  I  am  informed  that  the  German  opti- 
cian, Nobert,  has  ruled  bands  containing  about  224,000 
lines  to  the  inch.  He  regularly  makes  plates  with 
bands  consisting  of  from  about  11,000  to  112,000  lines 


18 


MOLECULES  AND  AVOGADRO'S   LAW. 


to  the  inch.  These  bands  are  numbered  from  the  1st 
to  the  19th,  and  are  used  for  microscopic  tests.  I  am 
indebted  to  our  friend  Mr.  Stodder  for  the  opportu- 
nity of  exhibiting  to  you  a  beautiful  photograph  of  the 
19th  band,  containing  over  112,000  lines  to  the  inch 
(Fig,  3).  The  photograph  was  made  with  one  of  Tolles's 


.  3.—  Nobert's  19th  Band. 


microscopes,  and  any  microscopist  will  tell  you  that  to 
resolve  this  band  is  a  great  triumph  of  art,  and  that 
you  could  have  no  better  evidence  of  the  skill  of  our 
eminent  optician  than  this  photograph  affords.  In 
projecting  the  image  on  the  screen,  some  of  the  sharp- 
ness is  lost,  but  I  think  the  separate  lines  of  the  band 
must  be  distinctly  visible  to  all  who  are  not  too  far  off. 
]STow,the  distance  between  the  lines  on  the  original 
plate  is  not  very  different  from  one-half  of  the  mean 
length  of  a  wave  of  violet  light,  or  one-third  of  a  wave- 
length of  red  light  ;  and,  what  is  still  more  to  the  pur- 
pose, these  very  bands  give  us  the  means  of  measuring 
the  dimensions  of  the  waves  of  light  themselves.  Evi- 
dently, then,  the  dimensions  with  which  we  are  dealing 
are  not  only  conceivable,  but  wholly  within  the  range 


THE   INTERSPACES   IN   GLASS.  19 


of  our  perceptions,  aided  as  they  have  been  by  the  ap- 
pliances of  modern  science. 

But,  to  return  to  my  argument :  these  values,  if 
they  are  not  wave-lengths,  are  real  magnitudes,  which 
differ  from  each  other  in  size  just  as  the  above  measure- 
ments show.  Moreover,  we  have  reason  to  believe  that 
the  various  color-giving  rays  differ  in  nothing  else,  and 
it  is  certain  from  astronomical  evidence  that  they  all 
pass  through  the  celestial  spaces  with  the  same  velocity. 
Now,  when  a  beam  of  light  enters  a  mass  of  glass,  not 
only  does  its  velocity  diminish,  but,  what  is  more  re- 
markable, the  different  rays  assume  at  once  different 
velocities,  and,  according  to  the  well-known  principles 
of  wave-motion,  the  unequal  bending  that  results  is 
the  necessary  effect  of  the  unequal  change  in  velocity 
which  the  rays  experience.  But,  if  the  material  of  the 
glass  were  perfectly  homogeneous  throughout,  it  is  im- 
possible to  conceive,  either  on  the  wave  theory  or  any 
other  theory  of  light  we  have  been  able  to  form,  how 
a  mere  difference  in  size  in  what  we  now  call  the 
luminous  waves  should  determine  this  unequal  velocity 
with  the  accompanying  difference  of  refrangibility,  and 
the  fact  that  such  a  difference  is  produced  is  thought 
by  many  to  be  strong  evidence  that  there  is  not  an  ab- 
solute continuity  in  the  material ;  in  fine,  that  there  are 
interstices  in  the  glass,  although  they  are  so  small  that 
it  requires  the  tenuity  of  a  ray  of  light  to  detect  them. 

Still  we  cannot  make  our  conceptions  the  measure 
of  the  resources  of  Nature,  and  I,  therefore,  do  not 
attach  much  value  to  this  additional  evidence  of  the 
molecular  structure  of  matter.  But  the  importance  of 
these  optical  phenomena  lies  in  this,  that,  assuming 
the  other  evidence  sufficient,  they  give  us  a  rough 
measure  of  the  size  of  the  molecules.  For,  as  is  evident 


20  MOLECULES  AND  AVOGADRO'S  LAW. 

from  our  illustration  with  the  wire  meshes,  the  size  ot 
the  molecular  spaces  cannot  be  very  different  from  that 
of  the  waves  of  light.  Our  diagram  shows  that  the  red 
waves  are  only  halt*  as  long  again  as  the  violet,  and  if 
the  molecular  spaces  were,  say,  either  ten  thousand 
times  larger  or  ten  thousand  times  smaller  than  the 
mean  length,  the  glass  could  produce  no  appreciable 
difference  of  effect  on  the  different  colored  rays.  We 
are  thus  led  to  the  result  that,  if  the  glass  is  an  aggre- 
gate of  molecules,  the  magnitude  of  these  molecules'is 
not  very  different  from  the  mean  length  of  a  wave  of 
light.  Accepting  the  undulatory  theory  of  light,  we 
can  submit  the  question,  as  Sir  William  Thompson  has 
done,  to  mathematical  calculation ;  and  the  result  is  that, 
though  the  effects  of  dispersion  could  not  be  produced 
unless  the  size  of  the  molecules  were  far  less  than  that  of 
the  wave-lengths,  yet  it  is  not  probable  that  the  size  is 
less  than  say  ^^iinnr  of  an  inch. 

Before  closing  the  lecture,  allow  me  to  dwell,  for  a 
few  moments,  on  the  second  of  the  two  classes  of  facts 
for  which  I  have  already  bespoken  your  attention,  since 
they  confirm  the  results  we  have  just  reached,  in  a  most 
remarkable  manner.  Every  one  has  blown  soap-bub- 
bles, and  is  familiar  with  the  gorgeous  hues  which  they 
display.  Many  of  you  have  doubtless  heard  that  blow- 
ing soap-bubbles  may  be  made  more  than  a  pleasant 
pastime,  and  I  will  endeavor  to  show  how  it  can  be 
made  a  philosophical  experiment,  capable  of  teaching 
some  very  wonderful  truths.  It  is  almost  impossible 
to  show  the  phenomena  to  which  I  refer  to  a  large 
audience,  and  I  cannot,  therefore,  feel  any  confidence 
in  the  success  of  the  experiment  which  I  am  about  to 
try ;  but  I  will  show  how  you  can  all  make  the  experi- 

1  The  mean  distance  between  the  centres  of  contiguous  molecules. 


HOW   TO   MAKE  SOAP-BUBBLES.  21 

ment  for  yourselves.  And,  first,  I  must  tell  you  how 
to  prepare  the  soap-suds. 

Procure  a  quart-bottle  of  clear  glass  and  some  of  the 
best  white  castile-soap  (or,  still  better,  pure  palm-oil 
soap).  Cut  the  soap  (about  four  ounces)  into  thin  shav- 
ings, and,  having  put  them  into  the  bottle,  fill  this  up 
with  distilled  or  rain-water,  and  shake  it  well  together., 
Repeat  the  shaking  until  you  get  a  saturated  solution 
of  soap.  If,  on  standing,  the  solution  settles  perfectly 
clear,  you  are  prepared  for  the  next  step ;  if  not,  pour 
off  the  liquid  and  add  more  water  to  the  same  shav- 
ings, shaking  as  before.  The  second  trial  will  hardly 
fail  to  give  you  a  clear  solution.  Then  add  to  two 
volumes  of  soap-solution  one  volume  of  pure,  con- 
centrated glycerine. 

Those  who  are  near  can  see  what  grand  soap-bubbles 
we  can  blow  with  this  preparation.  The  magnificent 
colors  which  are  seen  playing  on  this  thin  film  of  water 
are  caused  by  what  we  call  the  interference  of  light.  The 
color  at  any  one  point  depends  on  the  thickness  of  the 
film,  and  by  varying  the  conditions  we  can  show  that 
this  is  the  case,  and  make  these  effects  of  color  more 
regular.  For  this  purpose  I  will  pour  a  little  of  the 
soap-solution  into  a  shallow  dish,  and  dip  into  it  the 
open  mouth  of  a  common  tumbler.  By  gently  raising 
the  tumbler  it  is  easy  to  bring  away  a  thin  film  of 
the  liquid  covering  the  mouth  of  the  glass.  You  can 
all  easily  make  the  experiment,  and  study  at  your  lei- 
sure the  beautiful  phenomena  which  this  film  presents. 
To  exhibit  them  to  a  large  audience  is  more  difficult, 
but  I  hope  to  succeed  by  placing  the  tumbler  before 
the  lantern  in  such  a  position  that  the  beam  of  light 
will  be  reflected  by  the  film  upon  the  screen,  and  then, 
on  interposing  a  lens,  we  have  at  once  a  distinct  image 


22  MOLECULES  AND  AVOGADRO'S  LAW. 

of  the  film.  Success  now  depends  on  our  keeping 
perfectly  still,  as  the  slightest  jar  would  be  sufficient 
to  break  this  wonderfully  delicate  liquid  membrane. 
See !  the  same  brilliant  hues  which  give  to  the  soap- 
bubble  its  beauty  are  beginning  to  appear  on  our  film5 
but  notice  that  they  appear  in  regular  bands,  crossing 
the  film  horizontally.  As  I  have  already  stated,  the 
color  at  any  point  depends  on  the  thickness  of  the 
film,  and,  as  it  is  here  held  in  a  vertical  position,  it  is 
evident  that  the  effect  of  gravity  must  be  to  stretch 
the  liquid  membrane,  constantly  thinning  it  out,  be- 
ginning from  the  upper  end — which,  however,  it  must 
be  remembered,  appears  on  the  screen  at  the  lower  end, 
since  the  lens  inverts  the  image — and  notice  that,  as 
the  -§lm,  becomes  thinner  and  thinner,  these  bands 
of  color  which  correspond  to  a  definite  thickness  move 
downward,  and  are  succeeded  by  others  corresponding 
,to  a  thinner  condition  of  the  film,  which  give  place 
to  still  others  in  their  turn.  These  colors  are  not 
pure  colors,  but  the  effect  is  produced  by  the  over- 
lapping of  very  many  colored  bands,  and,  in  order  to 
reduce  the  conditions  to  the  simplest  possible,  we  must 
use  pure  colored  light — monochromatic  light,  as  we 
call  it.  Such  a  light  can  be  produced  by  placing  a 
plate  of  red  glass  (colored  by  copper)  in  front  of  the 
lantern.  At  once  all  the  particolors  vanish  and  we 
have  merely  alternate  red  and  dark  bands.  Watch, 
now,  the  bands  as  they  chase  each  other,  as  it  were, 
over  the  film,  and  notice  that  already  new  bands  cease 
to  appear,  and  that  a  uniform  light  tint  has  spread  over 
the  upper  half  (lower  in  the  image)  of  the  surface. 
Now  comes  the  critical  point  of  our  experiment.  If 
the  film  is  in  the  right  condition  so  that  it  can  be 
stretched  to  a  sufficient  degree  of  tenuity,  this  light 


OPTICAL   EFFECTS   OF  SOAP-BUBBLES.  23 

tint  will  be  succeeded  by  a  gray  tint,  ....  and  there 
it  appears  in  irregular  patches  at  the  upper  border.  But 
in  an  instant  all  has  vanished,  for  the  film  has  broken, 
as  it  always  breaks,  soon  after  the  gray  tint  appears. 


FIG.  4.— Bands  on  Soap-film. 

Having  now  seen  the  phenomena,  you  will  be  bet- 
ter prepared  to  appreciate  the  strength  of  the  ar- 
gument to  which  I  now  have  to  ask  your  careful 
attention.  You  know  that  the  red  and  dark  bands 
seen  in  the  last  experiment,  when  we  used  the  red 
glass,  are  caused  by  the  interference  of  the  rays  of 
light,  which  are  reflected  from  the  opposite  surfaces 
of  the  film.  It  is  evident  that  the  path  of  the  rays  re- 
flected from  the  back  surface  must  be  longer  than  that 
of  those  reflected  from  the  front  surface  by  just  twice 
the  thickness  of  this  film  of  water ;  and,  as  Prof. 
Tyndall  has  so  beautifully  shown  you  in  the  course  of 
lectures  just  finished,  whenever  this  difference  of  path 
brings  the  crests  of  the  waves  of  one  set  of  rays  over 
the  troughs  of  the  second  set,  we  obtain  this  won- 
derful result — that  the  union  of  the  two  beams  of  light 
produces  darkness.  It  would,  at  first  sight,  seem  that 


24  MOLECULES  AND   AVOGADRO'S  LAW. 

such  a  result  must  be  produced  in  the  case  of  our 
film  whenever  its  thickness  is  equal  to  •£,  £ ,  f ,  J,  or  any 
odd  number  of  fourths  of  the  length  of  a  wave  of  red 
light,  and  this  would  be  the  case  were  it  not  for  the 
circumstance  that,  in  consequence  of  certain  mechani- 
cal conditions,  the  rays  of  light  reflected  from  the  back 
of  the  film  lose  one-half  of  a  wave-length  in  the  very 
act  of  reflection.  But,  without  entering  into  details, 
which  have  been  so  recently  and  so  beautifully  illus- 
trated in  this  place,  let  me  call  your  attention  to  this 
diagram,  which  tells  the  whole  story : 


ORDER  OF  BANDS. 

Eetardation  of  rays 
reflected  from  back- 
surface  of  film. 

Thickness  of  film  in  waves  of 
red  light  n^  of  an  inch. 

Gray  film  

i  wave- 

1            ' 

H 

2 

^ 
3 

H 

4 

4*r 

5 

length. 

Less  thai 
fc 

1    c 
1* 

H 
if 

2 
2* 

i  ^  wave- 

r| 
1 

rf 
4 

4 
fi. 
4 

1 

4 

ength. 
t 

Light  film  

First  dark  band  

First  light  band         

Second  dark  band 

Second  light  band  

Third  dark  band 

Third  light  band   .  . 

Fourth  dark  band  

Fourth  light  band  

You  thus  see  that  the  theory  of  lighfc  enables  us  to 
measure  the  thickness  of  the  film,  and  we  know  that 
where  that  gray  tint  appeared  in  our  experiment  the 
thickness  of  the  film  was  less  than  -J-  of  the  length  of 


a  wave  of  red  light,  or  less  than  T^-^TO  of  an  inch,  and 
no  wonder  that  the  film  broke  when  it  reached  such  a 
degree  of  tenuity  as  that. 

But,  having  followed  me  thus  far,  and  being  assured, 
as  I  hope  you  are,  that  we  are  on  safe  ground,  and  talking 
about  what  we  do  know,  your  curiosity  will  lead  you 
to  inquire  whether  we  can  stretch  the  film  any  farther. 

The  facts  are  that,  after  the  appearance  of  the  gray 
tint,  although  the  film  evidently  stretches  to  a  limited 


SEPARATING   THE   MOLECULES  OF  WATER.  25 

extent,  it  very  soon  breaks.  Practically,  then,  we  can- 
not stretch  it  beyond  this  point  to  any  great  extent ; 
but  why  not  ?  Theoretically,  if  the  material  of  water 
is  perfectly  homogeneous,  there  would  seem  to  be  no 
good  reason  why  it  should  not  be  capable  of  an  in- 
definite extension,  and  why  this  film  could  not  be 
stretched  to  an  indefinite  degree  of  attenuation.  As- 
sume, however,  that  water  consists  of  molecules  ot  a 
definite  size,  then  it  is  evident  that  a  limit  would  be 
reached  as  soon  as  the  thickness  of  the  film  was  re- 
duced to  the  diameter  of  a  single  molecule.  Obvi- 
ously we  could  not  stretch  the  film  beyond  this  with- 
out increasing  the  distance  between  the  molecules,  and 
thus  increasing  the  total  volume  of  the  water.  Now, 
there  is  evidence  that,  when  the  gray  tint  appears, 
we  are  approaching  a  limit  of  this  sort.  It  is  hardly 
necessary  to  say  that  we  cannot  separate,  to  any  con- 
siderable extent,  the  molecules  of  water  from  each 
other — that  is,  increase  the  distance  between  them — 
without  changing  the  liquid  into  a  gas,  or,  in  other 
words,  converting  the  water  into  steam,  and  the  only 
way  in  which  we  can  produce  this  effect  is  by  the 
application  of  heat.  The  force  required  is  enormous, 
but  the  force  exerted  by  heat  is  adequate  to  the  work, 
and  it  is  one  of  the  triumphs  of  our  modern  science 
that  we  have  been  able  to  measure  this  force,  and  re- 
duce it  to  our  mechanical  standard.  In  order  to  pull 
apart  the  molecules  of  a  pound  of  water,  that  is,  con- 
vert it  into  steam,  we  must  exert  a  mechanical  power 
which  is  the  equivalent  of  822,600  foot-pounds,  that 
is,  a  power  which  would  raise  nearly  four  tons  to  the 
height  of  one  hundred  feet,  and,  as  we  can  readily  esti- 
mate the  weight  of  say  one  square-inch  of  our  film,  we 
know  the  force  which  would  be  required  to  pull  apart 
the  molecules  of  which  it  consists. 

4 


26  MOLECULES  AND  AVOGADRO'S  LAW. 

Again,  on  the  other  hand,  singular  as  it  may  seem., 
we  have  been  able  to  calculate  the  force  which  is  re- 
quired to  stretch  the  film  of  water.  This  calculation  is 
based  on  the  theory  of  capillary  action,  of  which  the 
soap-bubble  is  an  example.  Moreover,  to  a  certain 
limit,  we  are  able  to  measure  experimentally  the  force 
required  to  stretch  the  film,  and  we  find  that,  as  far  as 
our  experiments  go,  the  theory  and  the  experiments 
agree.  Our  experiments  necessarily  stop  long  before 
we  reach  the  limit  of  the  gray  film ;  but  our  theory  is 
not  thus  limited,  and  we  can  readily  calculate  how 
great  a  force  would  be  required  to  stretch  the  film 
until  the  thickness  was  reduced  to  ^^T^O^^IT^  °*  an 
inch  ;  that  is,  the  -^Vo  of  the  thickness  of  the  light  film, 
or  the  y^-e-Q-Q  °f  a  wave-length.  Now,  the  force  required 
to  do  this  work  is  as  great  as  that  required  to  pull 
apart  the  molecules  of  the  water  and  convert  the  liquid 
into  vapor.  It  is  therefore  probable  that,  before  such  a 
degree  of  tenuity  can  be  attained,  a  point  would  be 
reached  where  the  film  had  the  thickness  of  a  single 
molecule,  and  that,  in  stretching  it  further,  we  should 
not  reduce  its  thickness,  but  merely  draw  the  molecules 
apart,  and,  thus  overcoming  the  cohesion  which  deter- 
mines its  liquid  condition,  and  gives  strength  to  the 
film,  convert  the  liquid  into  a  gas. 

There  are  many  other  physical  phenomena  which 
point  to  a  similar  limit,  and,  unless  there  is  some  fal- 
lacy in  our  reasoning,  this  limit  would  be  reached  at 
about  the  yo\TFthr,Tnnr  °^  an  ^nctl-  Moreover,  it  is  wor- 
thy of  notice  that  all  these  phenomena  point  to  very 
nearly  the  same  limit.  I  have  great  pleasure  in  refer- 
ring you,  in  this  connection,  to  a  very  remarkable  pa- 
per of  Sir  William  Thompson,  of  Glasgow,  on  this  sub- 
ject, which,  appearing  first  in  the  English  scientific 


DIMENSIONS   OF  MOLECULES.  27 

weekly  called  Nature,  was  reprinted  in  Silliman's 
Journal  of  July,  1870.  He  fixes  the  limits  at  between 

the  inro-^/uinr  and  tne  ir,Tnry,<foTjnnr  of  an  incn>  and? in 
order  to  give  some  conception  of  the  degree  of  coarse- 

grainedness  (as  he  calls  it)  thus  indicated  by  the  struct- 
ure, he  adds  that,  if  we  conceive  a  sphere  of  water  as 
large  as  a  pea  to  be  magnified  to  the  size  of  the  earth? 
each  molecule  being  magnified  to  the  same  extent,  the 
magnified  structure  would  be  coarser-grained  than  a 
heap  of  small  lead  shot,  but  less  coarse-grained  than  a 
heap  of  cricket-balls. 

These  considerations  will,  I  hope,  help  to  show  you 
how  definite  the  idea  of  the  molecule  has  become  in  the 
mind  of  the  physicist.  It  is  no  longer  a  metaphysical 
abstraction,  but  a  reality,  about  which  he  reasons  as 
confidently  and  as  successfully  as  he  does  about  the  plan- 
ets. He  no  longer  connects  with  this  term  the  ideas 
of  infinite  hardness,  absolute  rigidity,  and  other  in- 
credible assumptions,  which  have  brought  the  idea  of  a 
limited  divisibility  into  disrepute.  His  molecules  are 
definite  masses  of  matter,  exceedingly  small,  but  still 
not  immeasurable,  and  they  are  the  points  of  applica- 
tion to  which  he  traces  the  action  of  the  forces  with 
which  he  has  to  deal.  These  molecules  are  to  the  physi- 
cist real  magnitudes,  which  are  no  further  removed 
from  our  ordinary  experience  on  the  one  side,  than  are 
the  magnitudes  of  astronomy  on  the  other.  In  regard 
to  their  properties  and  relations,  we  have  certain  defi- 
nite knowledge,  and  there  we  rest  until  more  knowledge 
is  reached.  The  old  metaphysical  question  in  regard  to 
the  infinite  divisibility  of  matter,  which  was  such  a  sub- 
ject of  controversy  in  the  last  century,  has  nothing  to  do 
with  the  present  conception.  Were  we  small  enough 
to  be  able  to  grasp  the  molecules,  we  might  be  able  to 


28  MOLECULES  AND   AVOGADRO'S  LAW. 

split  them,  and  so,  were  we  large  enough,  we  might  be 
able  to  crack  the  earth ;  but  we  have  made  sufficient 
advance  since  the  days  of  the  old  controversy  to  know 
that  questions  of  this  sort,  in  the  present  state  of  knowl- 
edge, are  both  irrelevant  and  absurd.  The  molecules 
are  to  the  physicist  definite  units,  in  the  same  sense 
that  the  planets  are  Tinits  to  the  astronomer.  The  ge- 
ologist tears  the  earth  to  pieces,  and  so  does  the  chem- 
ist deal  with  the  molecules,  but  to  the  astronomer  the 
earth  is  a  unit,  and  so  is  the  molecule  to  the  physicist. 
The  word  molecule,  which  means  simply  a  small  mass 
of  matter,  expresses  our  modern  conception  far  better 
than  the  old  word  atom,  which  is  derived  from  the 
Greek  a,  privative,  and  re//,z/6>,  and  means,  therefore,  in- 
divisible. In  the  paper  just  referred  to,  Sir  W.  Thomp- 
son used  the  word  atom  in  the  sense  of  molecule,  and 
this  must  be  borne  in  mind  in  reading  his  article.  We 
shall  give  to  the  word  atom  an  utterly  different  signifi- 
cation, which  we  must  be  careful  not  to  confound  with 
that  of  molecule.  In  our  modern  chemistry,  the  two 
terms  stand  for  wholly  different  ideas,  and,  as  we  shall 
see,  the  atom  is  the  unit  of  the  chemist  in  the  same 
sense  -that  the  molecule  is  the  unit  of  the  physicist. 
But  we  will  not  anticipate.  It  is  sufficient  for  the  pres- 
ent if  we  have  gained  a  clear  conception  of  what  the 
word  molecule  means,  and  I  have  dwelt  thus  at  length 
on  the  definition  because  I  am  anxious  to  give  you  the 
same  clear  conviction  of  their  existence  which  I  have 
myself.  As.  I  have  said  before,  they  are  to  me  just  as 
much  real  magnitudes  as  the  planets,  or,  to  use  the 
words  of  Thompson,  "  pieces  of  matter  of  measurable 
dimensions,  with  shape,  motion,  and  laws  of  action,  in- 
telligible subjects  of  scientific  investigation." 

1  See  Lecture  on  Molecules,  by  Prof.  Maxwell,  Nature,  Sept,  25^  1873. 


LECTUEE  H. 

THE    MOLECULAR    CONDITION    OF    THE     THREE    STATES     OF 
MATTER THE   GAS,  THE   LIQUID,  AND   THE   SOLID. 

IN  my  first  lecture  I  endeavored  to  give  you  some 
conception  of  the  meaning  of  the  word  molecule,  and  this 
meaning  I  illustrated  by  a  number  of  phenomena,  which 
not  only  indicate  that  molecules  are  real  magnitudes, 
but  which  also  give  us  some  idea  of  their  absolute  size. 

Avogadro's  law  declares  that  all  gases  contain,  un- 
der like  conditions  of  temperature  and  pressure,  the 
same  number  of  molecules  in  the  same  volume ;  and, 
if  we  can  rely  on  the  calculations  of  Thompson,  which 
are  based  on  the  w ell-known  theorem  of  molecular  me- 
chanics deduced  by  Clausius,  this  number  is  about  one 
hundred  thousand  million  million  million,  or  1C23  to  a 
cubic  inch.  Of  course,  as  the  volume  of  a  given  quan- 
tity of  gas  varies  with  its  temperature  and  pressure,  the 
number  of  molecules  contained  in  a  given  volume  must 
vary  in  the  same  way;  and  the  above  calculation  is 
based  on  the  assumption  that  the  temperature  is  at  the 
freezing-point,  and  the  pressure  of  the  air,  as  indicated 
by  the  barometer,  thirty  inches.  The  law  only  holds, 
moreover,  when  the  substances  are  in  the  condition  of 
perfect  gases.  It  does  not  apply  to  solids  or  liquids, 
and  not  even  to  that  half-way  state  between  liquids  and 
gases  which  Dr.  Andrews  has  recently  so  admirably 


30 


THE   THREE  STATES   OF  MATTER. 


defined.  In  the  state  of  perfect  gas,  it  is  assumed  that 
the  molecules  are  so  widely  separated  that  they  exert 
no  action  upon  each  other,  but  the  moment  the  gas  is 
so  far  condensed  ik«t  the  molecules  are  brought  within 
the  sphere  of  their  mutual  attraction,  then,  although  the 
aeriform  state  is  still  retained,  we  no  longer  find  that 

the  law  rigidly  holds ;  and 
when,  by  the  condensation, 
the  state  of  the  substance  is 
changed  to  that  of  a  liquid 
or  a  solid,  all  traces  of  the 
law  disappear.  In  order 
that  you  may  gain  a  clear 
conception  of  this  relation, 
I  shall  ask  your  attention  in 
this  lecture  to  the  explana- 
tion which  our  molecular 
theory  gives  of  the  char- 
acteristic properties  of  the 
three  conditions  of  matter 
the  gas,  the  liquid,  and  the 
solid.  We  begin  with  the 
gas,  because  its  mechanica 
condition  is,  theoretically  ai 
least,  by  far  the  simplest  of 
the  three. 

Every  one  of  my  audi- 
ence must  be  familiar  with 


the  fact  that  every  gas  is  in 
a  state  of  constant  tension 
tending  to  expand  indefi- 
nitely into  space.  In  the 
case  of  our  atmosphere,  this  tension  is  so  great  that  the 
air  at  the  level  of  the  sea  exerts  a  pressure  of  between 


FIG.  5. — Barometer. 


EXPANSIVE  ENERGY  IN  GASES.  31 

fourteen  and  fifteen  pounds  on  every  square  inch  of 
surface — about  a  ton  on  a  square  foot. 

It  is  this  pressure  which  sustains  the  column  of 
mercury  in  the  tube  of  a  barometer  (Fig.  5);  and  since, 
by  the  laws  of  hydrostatics,  the  height  of  this  column 
of  mercury  depends  on  the  pressure  of  the  air,  rising 
and  falling  in  the  same  proportion  as  the  pressure  in- 
creases or  diminishes,  we  use  the  barometer  as  a  meas- 
ure of  the  pressure,  and,  instead  of  estimating  its  amount 
as  so  many  pounds  to  the  square  inch,  we  more  fre- 
quently describe  it  by  the  height  in  inches  (or  centi- 
metres) of  the  mercury-column,  which  it  is  capable  of 
sustaining  in  the  tube  of  a  barometer.  The  tension  of 
the  air  is  balanced  by  the  force  of  gravitation,  in  con- 
sequence of  which  the  lower  stratum  of  the  air  in  which 
we  live  is  pressed  upon  by  the  whole  weight  of  the  su- 
perincumbent mass.  The  moment,  however,  the  ex- 
ternal pressure  is  relieved,  the  peculiar  mechanical  con- 
dition of  the  gas  becomes  evident. 

Hanging  under  this  large  glass  receiver  is  a  small 
rubber  bag  (a  common  toy  balloon),  partially  dis- 
tended with  air  (Fig.  6).  The  air  confined  within  the 
bag  is  exerting  the  great  tension  of  which  I  have  spo- 
ken, but  the  mass  remains  quiescent,  because  this  ten- 
sion is  exactly  balanced  by  the  pressure  of  the  atmos- 
phere on  the  exterior  surface  of  the  bag.  You  see,  how- 
ever, that,  as  we  remove,  by  means  of  this  air-pump, 
the  air  from  the  receiver,  and  thus  relieve  the  external 
pressure,  the  bag  slowly  expands,  until  it  almost  com- 
pletely fills  the  bell.  There  can,  then,  be  no  doubt  that 
there  exists  within  this  mass  of  gas  a  great  amount  of 
energy,  and  since  this  energy  exactly  balances  the  at- 
mospheric pressure,  it  must  be  equal  to  that  pressure. 

But  I  wish  to  show  you  more  than  this,  for  not  only 


32  THE   THREE   STATES   OF  MATTER. 

is  it  true  that  the  bag  expands  as  the  pressure  is  relieved, 
but  it  is  also  true  that  the  gas  in  the  bag  expands  in 
exactly  the  same  proportion  as  the  external  pressure 


FIG.  6. — Expanding  Bag  under  Air-pump. 

diminishes.  In  order  to  prove  this,  I  will  now  place 
under  this  same  glass  one  of  those  small  gasometers, 
which  are  used  by  the  itinerant  showmen  in  our  streets 
for  measuring  what  they  call  the  volume  of  the  lungs, 
while  under  this  tall  bell  at  the  side  I  have  arranged  a 
barometer-tube  for  measuring  the  external  pressure. 
The  two  receivers  are  connected  together  by  rubber 
hose,  so  as  to  form  essentially  one  vessel,  and  both  are 
connected  with  the  air-pump. 

We  will  begin  by  blowing  air  into  the  gasometer 
until  the  scale  marks  100  cubic  inches,  and,  noticing 
after  adjusting  the  apparatus  that  the  barometer  stands 
at  30  inches,  we  will  now  proceed  to  exhaust  the  air,  at 
the  same  time  carefully  watching  the  barometer.  .  .  . 
It  has  now  fallen  to  15  inches ;  that  is,  the  pressure  on 


LAW   OF   MARIOTTE.  33 

the  outside  of  the  gasometer  has  been  reduced  to  one- 
half,  and  the  scale  of  the  instrument  shows  me  that  the 
volume  of  the  air  in  the  interior  has  become  200  cubic 
inches ;  that  is,  has  doubled.  But  let  us  continue  the 
exhaustion.  .  .  .  The  barometer  now  marks  10  inches,, 
showing  that  the  pressure  has  been  reduced  to  one- 
third.  The  gasometer  now  contains  300  cubic  inches 
of  gas.  The  volume,  then,  has  trebled.  .  .  .  Pushing 
the  experiment  still  further,  we  have  now  the  barome- 
ter standing  at  7-J  inches,  and  the  scale  of  the  gasome- 
ter shows  that  the  volume  of  the  inclosed  air  has  be- 
come 400  cubic  inches.  The  pressure  has  been  reduced 
to  one-fourth,  and  the  volume  of  the  air  has  quadrupled  ; 
and  so  we  might  go  on.  .  .  .  Let,  now,  the  atmosphere 
reenter  the  apparatus,  and  at  once  the  air  in  the  gas- 
ometer shrinks  to  its  original  volume,  while  the  barome- 
ter goes  back  to  30  inches. 

We  might  next  take  a  condensing-pump,  and,  ar- 
ranging our  apparatus  so  as  to  resist  the  ever-increasing 
pressure,  as  the  air  was  forced  into  the  receivers,  we 
should  find  that,  when  the  barometer  marked  60  inches, 
the  scale  of  the  gasometer  would  show  50  cubic  inches, 
and  that,  when  the  mercury  column  had  risen  to  120 
inches,  the  air  in  the  gasometer  would  have  shrunk  to 
25  cubic  inches ;  and  so  on.  There  are,  however,  ob- 
vious mechanical  difficulties,  which  make  this  phase  of 
the  experiment  unsuitable  for  a  large  lecture-room,  and 
what  we  have  seen  is  sufficient  to  illustrate  the  general 
principle  which  I  wished  to  enforce.  The  principle, 
in  a  few  words,  is  this  : 

The  volume  of  a  confined  mass  of  gas  is  inversely  pro- 
portional to  the  pressure  to  which  it  is  exposed :  the 
smaller  the  pressure  the  larger  the  volume,  and  the 
greater  the  pressure  the  less  the  volume. 


34  THE   THREE   STATES  OF  MATTER. 

This  principle  holds  true  not  only  with  air,  bnt 
also  with  every  kind  of  aeriform  matter.  If,  instead 
of  using  that  mixture  of  oxygen  and  nitrogen  we  call 
air,  we  had  introduced  into  the  gasometer  100  cubic 
inches  of  pure  oxygen  or  of  pure  nitrogen,  or  of  any 
other  true  gas,  we  should  have  obtained  precisely  the 
same  effect.  The  results  of  the  experiment  are  not  in 
the  least  degree  influenced  by  the  nature  of  the  gas 
employed  ;  and,  assuming  that  we  start  with  the  same 
gas- volumes,  the  resulting  volumes  are  the  same  at  each 
stage  of  the  experiment.  In  every  case  the  volume 
varies  inversely  as  the  pressure.  The  principle  thus 
developed  is  one  of  the  most  important  laws  of  physical 
science.  It  was  discovered  by  the  chemist  Boyle  in 
England  in  1662,  and  verified  by  the  Abbe  Mariotte 
in  France  somewhat  later,  and  is  by  some  called  the 
law  of  Mariotte,  and  by  others  the  law  of  Boyle. 

It  is  always  important  to  look  at  the  phenomena  of 
Nature  from  different  sides,  for  otherwise  we  shall  be 
liable  to  mistake  their  true  relations  when  we  see  them 
under  unusual  aspects.  So,  in  order  that  we  may  the 
more  fully  comprehend  the  bearing  of  the  law  of  Mari- 
otte on  the  philosophy  of  chemistry,  it  will  be  well  for 
us  to  study  this  important  principle  from  a  point  of 
view  somewhat  different  from  that  we  have  just  pre- 
sented. 

Both  in  the  rubber  bag  and  in  the  small  gasometer 
we  experimented  with  the  constant  quantity  of  gas 
which  we  at  first  introduced,  and  we  measured  its  vary- 
ing volume  with  the  changing  pressure.  But  more 
frequently  we  have  to  deal  with  a  constant  volume  of 
gas,  and  to  consider  what  quantity  of  gas — measured  by 
its  weight — a  given  vessel  holds  under  different  press- 
ures. Here  is  a  strong  copper  reservoir  holding,  we 


TENSION  AND  PRESSURE.  35 

will  assume,  a  cubic  foot  of  gas,  and,  excepting  the  very 
small  fluctuations  caused  by  variations  of  temperature, 
this  volume  is  constant.  Connected  with  the  reservoir 
is  a  pressure-gauge  similar  to  those  you  may  see  on  any 
steam-boiler,  and  by  this  we  can  measure  the  tension 
of  the  confined  gas.  By  means  of  this  pump  we  can 
force  air  or  any  other  gas  into  the  chamber,  and  as  we 
work  the  pump  our  gauge  shows  an  ever-increasing 
tension  ;  and  here,  lest  you  should  be  confused  by  the 
two  terms  tension  and  pressure  applied  to  the  same 
manifestation  of  energy,  let  me  call  your  attention  to  the 
obvious  distinction  between  the  condition  of  permanent 
elasticity  or  tension  of  a  mass  of  gas,  and  either  the 
outward  pressure  which  in  consequence  of  its  tension 
the  gas  exerts  on  every  surface  exposed  to  its  action,  or 
the  external  pressure  by  which  the  tension  is  balanced 
and  the  mass  of  gas  confined  within  a  limited  volume. 
Still,  as  in  a  state  of  rest,  the  tension  everywhere  ex- 
actly balances  the  pressure,  the  two  terms  are  frequently 
interchangeable,  although  it  is  usual  to  estimate  pressure 
as  so  many  pounds  on  a  square  inch,  and  to  measure 
tension  by  the  height  of  the  column  of  mercury  which 
it  is  capable  of  sustaining.  Either  of  these  measures, 
however,  can  always  be  easily  reduced  to  the  other. 

Now,  what  relation  does  the  tension  of  the  air  in 
this  copper  vessel  sustain  to  the  quantity  of  air  (meas- 
ured, of  course,  by  its  weight)  which  the  chamber  con- 
tains ?  The  law  of  Mariotte,  as  we  have  already  stated 
it,  enables  us  to  answer  this  question.  We  already 
know  that  if  we  force  two  cubic  feet  of  air  into  one 
cubic  foot,  the  pressure  exerted  on  this  mass  of  gas,  and 
therefore  the  tension  of  the  gas,  must  be  doubled.  If 
we  force  three  cubic  feet  into  one  cubic  foot,  both  the 
pressure  and  the  tension  must  be  trebled,  etc.  In  other 


36  THE   THREE  STATES   OF   MATTER. 

words,  the  tension  of  a  mass  of  gas  confined  under  a 
constant  volume  will  be  proportional  to  the  quantity — 
that  is,  to  the  weight — of  gas  so  confined,  and  con- 
versely the  weight  must  be  proportional  to  the  tension. 
But,  as  you  see,  this  is  merely  another  mode  of  stating 
the  same  general  property  of  aeriform  matter  which  we 
have  called  the  law  of  Mariotte.  If  it  is  true  that 
the  volume  of  a  constant  weight  of  any  gas  is  inversely 
proportional  to  the  pressure  to  which  it  is  exposed,  it 
must  also  be  true  that  the  tension  of  a  constant  volume 
of  the  same  gas  is  directly  proportional  to  its  weight. 

Consider  a  further  consequence  of  the  property  of 
aeriform  matter  we  have  been  discussing,  which  exhibits 
still  another  phase  of  the  law  of  Mariotte.  According 
to  the  well-known  principle  of  Archimedes,  every  ob- 
ject immersed  in  the  atmosphere  is  buoyed  up  by  a 
force  exactly  equal  to  the  weight  of  air  it  displaces. 
This  force,  which  produces  such  a  marked  effect  in  the 
ascension  of  a  balloon,  cannot  be  neglected  in  any  sci- 
entific investigation  in  which  it  becomes  necessary  to 
determine  weights  with  great  accuracy.  It  is,  however, 
a  variable  force,  because,  since  the  tension  of  the  atmos- 
phere as  shown  by  the  barometer  is  continually  varying, 
the  weight  of  air  displaced  in  any  case  must  also  vary. 
But,  working  as  we  must  amid  this  variableness,  the 
law  of  Mariotte  comes  to  our  aid  and  enables  us  to  pre- 
dict what  must  be  the  effect  in  any  given  case  ;  for  as 
the  weight  of  a  constant  volume  of  gas  is  directly  pro- 
portional to  its  tension,  so  the  weight  of  air  displaced 
by  a  body  of  invariable  dimensions  must  be  propor- 
tional to  the  heights  of  the  barometer  column  which  at 
different  times  measures  the  tension  of  the  atmosphere. 

In  this  discussion  of  Mariotte's  law,  we  have  neces- 
sarily assumed  that  all  the  conditions  which  may  modify 


MOLECULAR  MOTION.  37 

the  volume  or  density  of  a  mass  of  gas  were  constant, 
except  only  the  one  we  have  been  studying.  It  is, 
however,  a  familiar  fact  that  the  condition  of  our  at- 
mosphere may  be  modified  by  several  causes,  and  of 
these  temperature  produces  even  a  greater  effect  than 
the  ordinary  variations  of  pressure.  To  the  influence 
of  temperature  on  the  condition  of  a  gas  we  must 
next  give  our  attention ;  but,  before  we  attack  this 
somewhat  difficult  problem,  let  me  point  out  to  you 
that  the  law  of  Mariotte  or  Boyle  is  most  closely 
related  to  the  law  of  Avogadro.  The  one  law  is 
found  to  hold  just  as  far  as  the  other,  and  any  de- 
viation from  the  one  is  accompanied  by  a  corre- 
sponding deviation  from  the  other.  So  close,  indeed, 
is  the  connection,  that  we  can  not  resist  the  convic- 
tion that  the  two  laws  are  merely  different  phases  of 
one  and  the  same  condition  of  matter ;  and  our  mole- 
cular theory  explains  this  connection  in  the  following 
way: 

The  molecules  of  a  body  are  not  isolated  masses  in 
a  fixed  position,  all  at  rest,  but,  like  the  planets,  they 
are  in  constant  motion.  The  greatest  length  of  path 
over  which  this  motion  can  ever  extend  must  be  ex- 
ceedingly short,  so  short,  indeed,  that,  if  the  path  could 
be  traced,  it  would  be  wholly  imperceptible  to  our 
senses,  even  when  aided  by  the  most  powerful  instru- 
ments. But,  nevertheless,  the  motion  is  none  the  less 
real,  and  none  the  less  capable  of  producing  mechanical 
effects.  In  a  gas  the  motions  of  the  molecules  are  sup- 
posed to  take  place  in  straight  lines,  the  molecules 
hurrying  to  and  fro  across  the  containing  vessel,  strik- 
ing against  its  walls,  or  else  encountering  their  neigh- 
bors, rebounding  and  continuing  on  their  course  in  a 
new  direction,  according  to  the  well-known  laws  which 


38  THE   THREE   STATES   OF  MATTER. 

govern  the  impact  of  elastic  bodies.  Of  course,  in  such 
a  system,  all  the  molecules  are  not  moving  with  the 
same  velocity  at  the  same  time  ;  but  they  have  a  cer- 
tain mean  velocity,  which  determines  what  we  call  the 
temperature  of  the  body,  and  the  higher  the  tempera- 
ture the  greater  is  this  mean  velocity ;  moreover,  the 
mean  velocity  of  the  molecules  of  each  substance  is 
always  the  same  at  the  same  temperature.  It  varies, 
however,  for  different  substances,  and,  for  any  given 
temperature,  the  less  the  density  of  the  gas  the  greater 
is  this  velocity,  although,  as  we  shall  hereafter  see,  the 
velocities  of  the  molecules  of  two  different  gases  are 
inversely  proportional,  not  simply  to  their  densities, 
but  to  the  square  roots  of  these  quantities.  We  are 
able  to  calculate  for  each  gas  at  least  approximately 
what  this  velocity  must  be  for  any  temperature,  and,  in 
the  case  of  hydrogen  gas,  the  value  at  the  temperature 
of  freezing  water  is  about  6,097  feet  per  second.  The 
internal  energy,  therefore,  in  a  pound  of  hydrogen  gas 
at  the  freezing-point  is  as  great  as  that  of  a  pound-ball 
moving  6,097  feet  per  second,  and  the  energy  in  an 
equal  volume  (a  little  over  6.6  cubic  yards  when  the 
barometer  is  at  30  inches)  of  any  other  true  gas  is 
equally  great  under  the  same  conditions;  a  greater 
molecular  weight  compensating  in  every  case  for  a  less 
molecular  velocity.  Let  us  now  bring  together  the 
two  remarkable  results  already  reached  in  this  lecture. 

One  cubic  inch  of  every  gas,  when  the  barometer 
marks  30  inches,  and  the  thermometer  32°  Fahr.,  con- 
tains 1023  molecules. 

Mean  velocity  of  hydrogen  molecules,  under  same 
conditions,  6,097  feet  per  second. 

It  is  evident,  then,  that  every  mass  of  gas  must 
contain  a  large  amount  of  internal  energy,  and  this 


WHAT   THERMOMETERS  TELL   US.  39 

energy  is  made  manifest  in  many  ways,  especially  in 
what  we  call  the  permanent  tension  of  the  gas.  Every 
surface  in  contact  with  a  mass  of  gas  is  being  con- 
stantly bombarded  by  the  molecules,  and  hence  the 
great  pressure  which  results.  JSTow,  the  greater  the 
number  of  molecules  in  a  given  space,  the  greater  will 
be  the  number  of  impacts  on  a  given  surface  in  a  given 
time,  and  therefore  the  greater  will  be  the  energy  of  the 
molecular  bombardment.  Evidently,  then,  according  to 
the  molecular  theory,  the  pressure  of  the  same  gas  on  a 
given  surface  ought  to  be  exactly  proportional  to  the  num- 
ber of  molecules  in  a  given  volume ;  or,  what  amounts 
to  the  same  thing,  to  the  weight  of  the  given  volume ; 
and  this  is  the  very  characteristic  property  of  aeriform 
matter,  which  we  have  called  the  law  of  Mariotte. 

Another  effect  of  molecular  motion  is  that  condi- 
tion of  matter  which  the  word  temperature,  just  used, 
denotes.  There  are  few  scientific  terms  more  difficult 
to  define  than  this  common  word  temperature.  In 
ordinary  language  we  apply  the  terms  hot  or  cold  to 
other  bodies  according  as  they  are  in  a  condition  to 
impart  heat  to,  or  abstract  it  from,  our  own,  and  the 
various  degrees  of  hot  or  cold  are  what  we  call,  in  gen- 
eral, temperature.  Two  bodies  have  the  same  temper- 
ature if,  when  placed  together,  neither  of  them  gives  or 
loses  heat;  and,  when,  under  the  same  conditions,  one 
body  loses  while  the  other  gains  heat,  that  body  which 
gives  out  heat  is  said  to  have  the  higher  temperature. 

Increased  temperature  tested  in  this  way  is  found 
to  be  accompanied  by  an  increase  of  volume,  and  we 
employ  this  change  of  volume  as  the  measure  of  tem- 
perature. This  is  the  simple  principle  of  a  thermome- 
ter. The  essential  part  of  this  instrument  is  a  glass 
bulb,  connected  with  a  fine  tube,  and  filled  with  mer- 


40  THE   THREE  STATES   OF   MATTER. 

cury  to  a  variable  point  in  the  stem.  The  least  change 
in  the  volume  of  the  mercury  is  indicated  by  the  rise 
of  the  column  in  the  tube.  Primarily,  the  thermome- 
ter is  a  very  delicate  measure  of  the  change  of  volume 
of  the  inclosed  liquid  ;  secondarily,  it  becomes  a  meas- 
ure of  temperature.  You  know  how  the  thermometer 
is  graduated.  We  plunge  it  into  a  mass  of  melting  ice 
and  mark  the  point  to  which  the  mercury  falls,  and 
then  we  immerse  it  in  free  steam,  and  mark  the 
point  to  which  the  column  rises.  We  now  divide  the 
distance  between  these  fixed  points  into  an  arbitrary 
number  of  equal  spaces,  and  continue  the  divisions  of 
the  same  size  above  and  below  our  two  standard  points. 
In  our  common  Fahrenheit  scale  this  distance  is  di- 
vided into  180  parts,  the  freezing-point  is  marked  32°, 
and  the  boiling,  of  course,  212° ;  the  zero  of  this  scale  be- 
ing placed  at  the  thirty-second  division  below  the  freez- 
ing-point. In  our  laboratories  we  generally  use  a  scale 
in  which  this  distance  is  divided  into  100  parts,  and 
the  freezing-point  marked  0°,  the  divisions  below  freez- 
ing being  distinguished  with  a  minus-sign.  All  this, 
however,  is  purely  arbitrary,  and  the  instrument  mere- 
ly gives  us  the  means  of  comparing  temperatures. 
Here,  for  example,  are  two  bodies.  We  apply  the 
thermometer  first  to  one  and  then  to  the  other.  It 
rises  in  each  case  to  50°.  The  only  information  we 
have  obtained  is,  that  both  bodies  are  at  the  same  tem- 
perature corresponding  to  a  certain  volume  of  the  mer- 
cury in  our  thermometer,  a  temperature  which  we  have 
agreed  to  call  50°  ;  and  we  can  predict  that,  if  the  two 
bodies  are  brought  together,  no  heat  will  pass  from  one 
to  the  other.  We  now  apply  the  thermometer  to  a 
third  body,  and  it  rises  to  100°.  We  thus  learn, 
further,  that  the  third  body  is  at  a  higher  temperature 


AN  ABSOLUTE   SCALE   OF  TEMPERATURE. 


41 


than  the  other  two,  and  in  a  condition  to  transfer  to 
them  a  part  of  its  heat.  We  cannot,  however,  say 
that  its  temperature  is  twice  as  high,  or  that  it  has  any 
definite  relation  to  that  of  the  other  two  bodies. 

There  is,  however,  a  theoretical  way  of  measuring 
temperature,  which  appears  to  lead  to  something  more 
than  a  mere  arbitrary  comparison.  Let  us  assume  that 
we  have  a  cylindrical  tube,  closed  below,  but  open 
above  (Fig.  7).  Let  us  further  assume  that  the  air 


646' 


S73< 


273' 


273° 

200° 
150° 
100° 
50° 
0° 

-50° 
-100° 
-150° 
-200° 


0°  LJ-2730 
FIG.  7. 


982C 


523° 

392° 
302° 
212° 
122° 

32° 
0° 

-58° 

-148° 
-238° 
-328° 


0°   J-4590 

FIG.  7,  bis. 


671' 


491° 
459° 


in  the  tube  is  confined  by  a  piston,  which  has  no 
weight,  and  moves  without  friction.  As  the  tempera- 
ture rises  or  falls,  of  course  our  assumed  piston  would 
rise  or  fall  in  the  tube,  following  the  expanding  or  con- 
tracting of  the  confined  air.  Let  us  mark  the  point  to 
which  the  piston  falls  at  the  temperature  of  freezing 


42  THE   THREE   STATES   OF  MATTER. 

water,  0°,  and  the  point  to  which  it  rises  at  the 
temperature  of  boiling  water,  100°.  Lastly,  let  us 
divide  the  distance  between  these  two  points,  as  in  a 
centigrade  thermometer,  into  one  hundred  equal  parts, 
and  continue  the  divisions  of  the  same  size  above  100° 
and  below  0°.  We  shall  find  that  we  can  make  almost 
exactly  273  such  divisions  before  reaching  the  closed 
bottom  of  our  tube.  Transfer,  now,  the  zero  of  our 
scale  to  this  lowest  point  or  bottom  of  our  tube,  so 
that  our  old  zero,  or  freezing-point  of  water,  will  be 
at  273°  of  the  new  scale,  and  the  boiling-point  of 
water  at  373°. 

Wo  shall  then  have  what  is  probably  very  nearly 
an  absolute  scale  of  temperature,  such  a  one  that  we 
can  say,  for  example,  that  the  temperature  at  500°  is 
twice  as  great  as  that  at  250°.  Moreover,  this  is  a 
scale  such  that  the  volume  of  any  gas,  under  the  same 
pressure,  is  exactly  proportional  to  the  temperature : 
for  example,  the  volume  of  a  given  mass  of  air  at  600° 
is  twice  as  great  as  the  volume  at  300°.  That  this 
must  be  the  case  for  air  is  evident  from  the  construc- 
tion of  our  theoretical  thermometer ;  and  it  is  equally 
true  of  any  other  perfect  gas,  for  there  would  be  no  dif- 
ference in  effect  whatever  if  the  tube  were  filled  with 
hydrogen,  oxygen,  or  nitrogen,  instead  of  air.  It  is 
very  easy  to  refer  degrees  of  our  ordinary  thermometer 
to  degrees  of  this  absolute  scale.  If  the  degrees  are 
centigrade,  we  have  merely  to  add  273  ;  if  they  are 
Fahrenheit,  we  must  add  459  (see  Fig.  7,  bis)  ;  and,  for 
many  purposes,  it  is  exceedingly  convenient  to  measure 
temperature  in  this  way.  Suppose,  for  example,  we  have 
100  cubic  inches  of  gas,  at  4°  centigrade,  and  we  wish  to 
know  what  would  be  its  volume  at  281°.  Converting 
these  values  into  absolute  degrees  by  adding  273,  we 


THE   LAW   OF  CHARLES.  43 

obtain  277°  and  554°.  Then,  since  the  volume  of  a  gas 
is  exactly  proportional  to  the  absolute  temperature,  we 
have  277  :  554  =  100  :  answer,  200  cubic  inches.  But 
the  chief  value  of  this  method  of  measuring  temperature 
is  to  be  found  in  the  simplicity  with  which  it  presents 
to  us  the  property  of  gases  we  have  been  studying.  The 
volume  of  a  gas  depends  solely  on  twro  conditions  :  its 
pressure  and  its  absolute  temperature.  As  I  before 
showed,  it  is  inversely  proportional  to  the  pressure, 
and  it  now  appears  that  it  is  directly  proportional  to 
the  absolute  temperature.  We  must  then  qualify  the 
law  of  Mariotte  by  a  second  principle,  equally  funda- 
mental and  important : 

The  volume  of  a  given  w.ass  of  gas,  under  a  constant 
pressure,  varies  directly  as  the  absolute  temperature. 

This  we  call  the  law  of  Charles. 

As  with  the  law  of  Mariotte,  so  with  the  law  of 
Charles,  we  shall  more  fully  comprehend  the  funda- 
mental relations  of  aeriform  matter,  of  which  either  law 
is  only  a  partial  expression,  if  we  study  the  subject  from 
a  somewhat  different  point  of  view.  Both  solids  and 
liquids  expand,  when  heated,  with  an  irresistible  force, 
but  a  mass  of  gas,  if  confined  in  a  suitable  vessel,  may 
be  heated  or  cooled  indefinitely  without  any  other 
change  of  volume  than  the  very  small  alteration  which 
che  vessel  itself  sustains.  Under  such  conditions,  the 
only  considerable  effect  produced  is  an  increase  or  dimi- 
nution of  the  tension  of  the  gas,  according  as  the  tem- 
perature rises  or  falls ;  and  by  connecting  with  the  ves- 
sel a  manometer  the  variation  of  tension  may  be  accu- 
rately measured. 

Assume  now  that  we  have  a  vessel  arranged  as  just 
described,  and  that  we  start  from  the  temperature  of 
melting  ice,  the  zero  degree  of  the  centigrade  scale,  we 


44  THE   THREE   STATES   OF  MATTER. 

shall  find — whatever  may  be  the  nature  of  the  gas  con- 
fined in  the  vessel,  and  whatever  may  bs  the  initial  ten- 
sion of  the  gas — that  this  tension  increases  by  ^-fg-  of  its 
value  at  0°  for  every  degree  through  which  the  tem- 
perature of  the  vessel  is  raised.  Hence,  at  273°  centi- 
grade the  tension  would  be  doubled,  at  546°  it  would 
be  trebled,  and  so  on.1  If,  next,  returning  to  our  start- 
ing-point, we  cool  the  vessel,  we  shall  find  that  the  ten- 
sion diminishes  by  -^  of  the  initial  value  for  every 
degree  of  temperature  lost ;  and,  although  we  have  not 
been  able  to  push  our  experiments  beyond  a  very  limited 
range  of  temperature,  yet,  if  the  law  observed  within 
this  range  holds  to  the  end,  it  is  obvious  that  at  —273° 
the  gas  would  have  lost  all  its  tension  and  would  exert 
no  pressure  whatever  on  the  interior  surface  of  the  ves- 
sel. Starting  now  from  this  point  of  no  tension,  the 
absolute  zero,  as  we  have  called  it,  and  raising  the  tem- 
perature of  the  vessel  one  degree,  we  should  develop  a 
small  amount  of  tension  (^\^  of  the  initial  tension  above 
mentioned),  and  each  additional  degree  of  temperature 
would  increase  the  tension  by  exactly  the  same  amount, 
so  that,  at  10°  above  the  absolute  zero,  the  tension  would 
be  ten  times  as  great ;  at  50°,  fifty  times  as  great ;  at  the 
100°,  one  hundred  times  as  great  as  at  1°,  and  so  on  to 
whatever  extent  we  may  raise  the  temperature.  In 
other  words,  the  tension  will  be  exactly  proportional  to 
the  absolute  temperature  ;  and  this  brings  us  to  another 
statement  of  the  law  of  Charles : 

1  The  f raction  ^T  would  be  exactly  correct  only  in  the  case  of  air  in- 
closed in  a  vessel  of  absolutely  constant  volume,  a  condition  which,  of 
course,  we  cannot  perfectly  command.  But  in  a  general  view  of  the  sub- 
ject we  may  leave  out  of  the  account,  not  only  the  small  expansion  of  the 
vessel  .above  referred  to,  but  also  certain  minute  diiferences  that  seem  to 
depend  on  the  imperfect  aeriform  condition  of  the  substances  with  which 
we  have  to  deal  in  our  actual  experiments. 


THE  LAW  EXPLAINED.  45 

The  tension  of  a  mass  of  gas  of  constant  volume 
varies  directly  as  the  absolute  temperature. 

But  the  same  conclusion  may  be  easily  deduced  from 
the  first  statement  of  the  same  law  by  a  simple  applica- 
tion of  the  cognate  law  of  Mariotte.  For  if,  as  an  ex- 
ample, we  conceive  of  a  mass  of  gas  whose  volume  has 
been  doubled  by  raising  the  temperature  from  0°  to 
273°,  and  then  consider  what  the  effect  must  be — ac- 
cording to  Mariotte's  law — if,  by  any  mechanical  means, 
the  volume  is  reduced  to  the  initial  state,  while  the 
temperature  is  maintained  at  273°,  it  will  be  obvious 
that  instead  of  a  double  volume,  we  shall  now  have  a 
double  tension ;  and,  since  the  final  state  is  the  same  as 
if  the  gas  had  been  heated  in  a  closed  vessel,  the  result  is 
precisely  that  which  the  second  statement  of  the  law  of 
Charles  predicts  ;  and  it  thus  appears  that  the  two  state- 
ments are  different  expressions  of  the  same  principle. 

The  molecular  theory  of  gases  explains  the  law  of 
Charles  very  much  in  the  same  way  as  it  explained 
the  law  of  Mariotte.  The  pressure  of  a  gas,  as  we  have 
seen,  is  due  to  its  molecular  energy.  If,  by  any  means, 
we  increase  that  energy,  we  must  also  increase  the 
pressure  in  the  same  proportion  ;  or,  if  the  gas  is  free 
to  expand  under  a  constant  pressure,  we  must  increase 
the  volume.  In  other  words,  the  effect  of  increased 
energy  must  be  the  same  as  the  effect  which  we  know 
follows  increased  temperature.  What  more  natural 
than  to  infer  that  the  unknown  condition,  to  which 
we  have  given  the  name  of  temperature,  is  simply 
molecular  energy  ?  Here,  then,  is  our  theoretical  ex- 
planation of  the  law  of  Charles.  The  temperature  of 
a  body  is  the  moving  power  of  its  molecules.  At  the 
0°  of  our  absolute  scale  the  molecules  would  be  re- 
duced to  a  state  of  rest,  and,  at  other  temperatures,  the 
molecular  energy  is  directly  proportional  to  the  de- 


46  THE   THREE   STATES   OF   MATTER. 

grees  of  this  scale  ;  so  that,  for  example,  the  molecules 
of  air,  at  273°  (the  0°  of  centigrade;,  have  only  one- 
half  of  the  energy  which  the  same  molecules  possess 
when  the  temperature  is  raised  to  546°.  As  the  press- 
ure exerted  by  the  air  must  be  proportional  to  the 
molecular  energy,  the  increased  temperature  will,  if 
the  air  is  confined,  double  this  pressure,  or,  if  the  air 
is  free  to  expand  under  the  constant  pressure  of  the 
atmosphere,  it  will  double  the  volume. 

It  would  lead  me  too  far  to  attempt  to  develop  here 
at  any  greater  length  the  dynamical  theory  of  heat, 
and  I  regret  that  I  am  not  able  to  do  more  than  to  give 
this  bare  outline  of  the  remarkable  properties  of  gases, 
which  it  so  beautifully  explains  ;  but  I  take  great  pleas- 
ure in  referring  all  who  are  interested  in  the  subject 
to  the  very  excellent  work  of  Prof.  Clerk  Maxwell 
on  the  theory  of  heat.  It  is  not  a  popular  work,  or 
one  which  is  easy  reading,  but  it  contains  a  most  ele- 
gant exposition  of  the  modern  theory  of  heat,  in  as 
simple  a  form  as  is  consistent  with  accuracy  and  con- 
ciseness. 

There  is  only  one  other  point,  in  connection  with 
the  molecular  theory  of  gases,  to  which  it  is  important 
for  me  to  refer  in  these  lectures.  We  have  seen  that 
all  gases  have  two  essential  characteristics  :  1.  Their 
volume  is  inversely  proportional  to  the  pressure  to  which 
they  are  exposed ;  and,  2.  Their  volume  is  directly 
proportional  to  the  absolute  temperature.  Now,  if  we 
assume  the  molecular  theory  of  gases  as  true,  it  can 
be  proved,  mathematically,  that  all  gases  at  the  same 
temperature  and  pressure  must  have  the  same  number 
of  molecules  in  the  same  volume.  The  proof  would 
be  out  of  place  here  ; l  but,  although  it  would  be  more 

1  Assume  that  we  have  two  entirely  similar  masses  of  different  gases 
— for  example,  oxygen  and  hydrogen — both  having  the  same  volume,  the 


MAXWELL'S   THEORY.  47 

satisfactory  to  enter  into  details,  I  shall  have  accom- 
plished the  first  object  of  this  lecture  if  I  have  been 

same  tension,  and  the  same  temperature.  Assume,  further,  that  these 
masses  are  brought  together  until  they  are  only  separated  by  an  elastic 
but  impenetrable  partition  which  will  freely  transmit  the  moving  power 
of  the  molecules  of  one  gas  to  those  of  the  other.  Under  such  conditions 
— to  the  extent  at  least  to  which  we  can  realize  them  experimentally — we 
know  that  there  would  be  no  change  whatever  in  the  tension  or  the  tem- 
perature of  the  similar  and  equal  volumes  of  gas,  provided  always  all  ex- 
ternal conditions  remained  unchanged ;  and  as,  by  assumption,  the  two 
masses  of  gas  are  perfectly  similar  in  all  their  external  relations  (in  re- 
spect, for  example,  to  the  nature  and  shape  of  the  vessels  they  fill),  it  is 
obvious,  not  only  that  in  the  initial  condition  the  total  moving  power  of 
the  molecules  of  one  mass  must  be  equal  to  the  total  moving  power  of 
the  molecules  of  the  other  mass,  but  also  that  this  relation  is  not  changed 
by  any  transmission  of  energy  resulting  from  an  interaction  of  the  mole- 
cules through  the  elastic  partition. 

Assume  next  that  the  partition  is  removed,  and  that  the  gases  are 
allowed  to  mix,  we  know  that  still  no  change  of  volume,  tension,  or  tem- 
perature would  result,  and  that  after  the  diffusion  was  complete  both  the 
tension  and  temperature  of  the  united  gas-volumes  would  remain  the 
same  as  before.  Moving  as  the  different  molecules  now  do  in  the  same 
space,  and  continually  colliding  with  each  other,  the  interaction  is  more 
rapid,  but  it  can  be  no  more  effectual  than  before  ;  and  it  is  therefore  obvi- 
ous that  after  the  diffusion  the  sum  of  the  moving  powers  of  the  mole- 
cules of  one  substance  must  still  remain  equal  to  the  sum  of  the  moving 
powers  of  the  molecules  of  the  other  substance. 

To  calculate  the  effect  of  the  collision  of  molecules,  under  the  con- 
ditions described  in  the  last  paragraph,  is  a  difficult  mathematical  prob- 
lem whose  happy  solution  was  one  of  the  most  important  contributions 
to  science  of  the  late  Prof.  Maxwell  He,  however,  has  shown  it  to  be 
a  necessary  deduction  from  the  well-known  principles  of  mechanics 
that  when  molecules,  like  those  of  oxygen  and  hydrogen  (regarding  them 
only  as  small  masses  of  matter  of  unequal  weight),  are  moving  in  the 
same  space  and  continually  colliding  with  each  other,  the  average  mov- 
ing power  of  the  molecules  of  one  kind  must  become  equal  to  the 
average  moving  power  of  the  molecules  of  the  other  kind,  and  this 
must  be  true  in  whatever  proportions  the  molecules  may  be  mixed. 
The  question,  it  must  be  noted,  is  wholly  one  of  averages;  for,  the 
moving  power  of  each  molecule  changes  at  every  collision,  and,  as 
the  collisions  must  succeed  each  other  with  an  exceeding  great  rapidity, 


48  THE  THREE   STATES  OF  MATTER. 

able  to  leave  with  you  a  clear  idea  of  the  three  laws 
which  may  be  said  to  define  the  aeriform  condition  of 
matter,  and  which  all  true  gases  obey — 

THE  LAW  OF  M  AKIOTTE, 
THE  LAW  OF  CHAKLES, 
THE  LAW  OF  AVOGADKO. 

Liquids  are  distinguished  from  gases  chiefly  in  hav- 
ing a  definite  surface.  Their  particles  have  the  same 
freedom  of  motion,  but  this  motion  is  limited  to  the 
mass  of  the  liquid.  The  particles  of  the  air,  if  uncon- 
fined,  would  move  off  indefinitely  into  space ;  but  the 
particles  of  this  water,  although  moving  with  equal 
freedom  within  the  liquid  mass,  cannot,  as  a  rule,  rise 
above  what  we  call  the  surface  of  the  water.  Again,  if 
we  introduce  a  quantity  of  air,  however  small,  into  a 
vacuous  vessel,  it  will  instantly  expand  until  it  com- 
pletely fills  the  vessel.  A  quantity  of  water,  under  the 
same  conditions,  will  fall  to  the  bottom  of  the  vessel, 
and  will  be  separated  by  a  distinct  surface  from  the 
vapor  which  forms  above  it.  Lastly,  if  a  gas  is  sub- 
jected to  pressure,  it  is  compressed  in  the  exact  pro- 
portion to  the  pressure,  while  with  a  liquid  the  com- 

the  condition  of  any  one  molecule  is  never  at  any  two  successive  moments 
the  same. 

It  must  be  true,  then,  in  the  case  of  our  assumed  equal  and  similar 
volumes  of  gas,  not  only  that  the  total  moving  power  of  all  the  molecules 
of  one  mass  is  equal  to  the  total  moving  power  of  all  the  molecules  of  tha 
other  mass,  but  also  that  on  the  average  the  moving  power  of  a  single 
molecule  of  one  gas  is  equal  to  the  moving  power  of  a  single  molecule 
of  the  other.  Obviously  this  necessarily  implies  that  the  number  of 
molecules  in  the  two  similar  and  equal  volumes  is  the  same,  and,  if 
the  same  in  two  similar  and  equal  volumes,  it  must  be  the  same  in 
two  equal  volumes,  whatever  be  the  shape  or  conditions  of  the  containing 
vessel. 


MOLECULAR  STRUCTURE   OF  LIQUIDS.  49 

pression  is  barely  perceptible,  even  when  the  press- 
ure is  exceedingly  great.  Hence,  gases  are  frequently 
called  compressible  and  liquids  incompressible  fluids. 

The  explanation  which  the  molecular  theory  gives 
of  this  difference  of  relations  is  very  simple.  In  the 
gas  the  molecules  are  separated  beyond  the  sphere  of 
each  other's  influence,  and  move  through  space  wholly 
free  from  the  effects  of  the  mutual  attraction.  In  a 
liquid,  on  the  other  hand,  this  attraction,  which  we  call 
cohesion,  is  very  sensible,  and  restrains  the  individual 
molecules  within  the  mass,  although  they  are  free  to 
move  among  themselves.  You  can  easily  understand, 
by  referring  again  to  the  diagram  (Fig.  2,  on  page  8), 
how  this  attractive  force  would  act. 

A  molecule,  in  the  midst  of  the  mass,  moves  freely, 
because  the  attractions  are  equal  in  all  directions,  but 
a  molecule  near -the  surface  is  in  a  very  different  con- 
dition. As  it  approaches  the  surface,  the  attraction 
toward  the  mass  of  the  liquid  becomes  greater  than  the 
attraction  toward  the  surface,  and  when  it  reaches  the 
surface  the  whole  force  of  the  inward  attraction  is  pulling 
it  back,  and,  unless  the  moving  power  of  the  molecule 
is  sufficiently  great  to  overcome  this  force,  its  motion 
is  arrested,  and  it  turns  back  on  its  course.  It  may 
happen,  however,  especially  when  heat  is  entering  the 
liquid,  that  some  of  the  molecules,  through  the  effects 
of  their  mutual  collisions,  acquire  sufficient  energy  to 
fly  off  from  the  liquid  mass,  and  hence  result  the  well- 
known  phenomena  of  evaporation.  Thus  our  theory 
defines  the  liquid  condition  of  matter,  and  explains  how 
the  liquid  is  converted  by  heat  into  the  gas. 

In  all  theoretical  discussions,  it  is  always  highly  sat- 
isfactory when,  in  following  out  our  theoretical  concep- 
tions to  their  consequences,  we  find  that  these  conse- 


50  THE  THREE   STATES  OF   MATTER. 

quences  are  actually  realized  in  natural  phenomena, 
and  such  satisfaction  we  can  have  in  the  present  case. 
Consider  what  must  be  the  form  which  a  mass  of  liquid 
molecules  isolated  in  space  would  necessarily  take.  Re- 
member that  these  molecules  are  moving  with  perfect 
freedom  within  the  body,  but  that  the  extent  of  the 
motion  of  each  molecule  is  limited  by  the  attraction  of 
the  mass  of  the  liquid.  Remember  also  that,  accord- 
ing to  the  well-known  principles  of  mechanics,  this  at- 
traction may  be  regarded  as  proceeding  from  a  single 
point,  called  the  centre  of  gravity.  Remember,  fur- 
ther, that  the  molecules  have  all  the  same  moving 
power,  and  you  will  see  that  the  extreme  limits  of  their 
excursions  to  and  fro  through  the  liquid  mass  must  be 
on  all  sides  at  the  same  distance  from  the  central  point. 
Hence  the  bounding  surface  will  be  that  whose  points 
are  all  equally  distant  from  the  centre.  I  need  not  tell 
you  that  such  a  surface  is  a  sphere,  nor  that  a  mass  of 
liquid  in  space  always  assumes  a  spherical  form.  The 
rain-drops  have  taught  every  one  this  truth.  Still,  a 
less  familiar  illustration  may  help  to  enforce  it.  I  have 
therefore  prepared  a  mixture  of  alcohol-and-water,  of 
the  same  specific  gravity  as  olive-oil,  and  in  it  I  have 
suspended  a  few  drops  of  the  oil.  By  placing  the  liquid 
in  a  cell,  between  parallel  plates  of  glass,  I  can  readily 
project  an  image  of  the  drops  on  the  screen,  and  I  wish 
you  to  notice  how  perfectly  spherical  they  are.  And  I 
would  have  you,  moreover,  by  the  aid  of  your  imagina- 
tion, look  within  this  external  form,  and  picture  to 
yourselves  the  molecules  of  oil  moving  to  and  fro 
through  the  drops,  but  always  slackening  their  motion 
where  they  approach  the  surface,  and  on  every  side 
coming  to  rest  and  turning  back  at  the  same  distance 
from  the  centre  of  motion. 


MOLECULAR  STRUCTURE  OF  SOLIDS.        51 

Neither  liquids  nor  gases  present  the  least  trace  of 
structure.  They  cannot  even  support  their  own  weight, 
much  less  sustain  any  longitudinal  or  shearing  stress. 
A  solid,  on  the  other  hand,  has  both  tenacity  and  struct- 
ure, and  resists,  with  greater  or  less  energy,  any  force 
tending  to  alter  its  form,  as  well  as  change  its  volumeo 
The  tenacity  and  peculiar  forms  of  elasticity  which 
solids  exhibit  are  characteristics  which  are  familiar  to 
every  one,  but  the  evidences  of  structure  are  not  so 
conspicuous.  The  structure  of  solids  is  most  frequently 
manifested  by  their  crystalline  form,  and  this  form  is 
one  of  the  most  marked  features  of  the  solid  state.  But 
although,  under  definite  conditions,  most  substances  as- 
sume a  fixed  geometrical  form,  yet,  to  ordinary  expe- 
rience, these  forms  are  the  exceptions,  and  not  the 
rule.  I  will  therefore  make  the  crystallization  of  solid 
bodies  the  subject  of  a  few  experimental  illustrations. 

For  the  first  experiment,  I  have  prepared  a  concen- 
trated solution  of  ammonic  chloride  (sal-ammoniac), 
and  with  this  I  will  now  smear  the  surface  of  a  small 
glass  plate.  Placing  this  before  our  lantern,  and  using 
a  lens  of  short  focus,  so  as  to  form  a  greatly-enlarged 
image  on  the  screen,  let  us  watch  the  separation  of  the 
solid  salt  as  the  solution  evaporates.  .  .  .  Notice  that, 
first,  small  particles  appear,  and  then  from  these  nuclei 
the  crystals  shoot  out  and  ramify  in  all  directions,  soon 
covering  the  plate  with  a  beautiful  net- work  of  the  fila- 
ments of  the  salt.  We  cannot  here,  it  is  true,  distin- 
guish any  definite  geometrical  form;  but  it  can  be 
shown  that  these  very  filaments  are  aggregates  of  such 
forms,  and  their  structure  is  made  evident  by  a  fact,  to 
which  I  would  especially  call  your  attention — that,  as 
the  crystalline  shoots  ramify  over  the  plate,  the  sprays 
keep  always  at  right  angles  to  the  stem,  or  else  branch 


52  THE   THREE   STATES   OF  MATTER. 

at  an  angle  of  45°,  which  is  the  half  of  a  right  angle 
(Fig.  8). 

For  a  further  illustration  of  the  process  of  crystal- 
lization I  have  prepared  a  solution  in  alcohol  of  a  solid 


FIG.  8. — Crystallization  of  Sal- Ammoniac. 


FIG.  9.— Crystallization  of  Urea. 


substance  called  urea,  with  which  we  will  experiment 
in  precisely  the  same  way  as  before.  .  .  .  The  process 
of  crystallization,  which  is  here  so  beautifully  exhibited, 
is  one  of  the  most  striking  phenomena  in  the  whole 
range  of  experimental  science.  It  is,  of  course,  not  so 
wonderful  as  the  development  of  a  plant  or  an  animal 
from  its  germ,  but  then  organic  growth  is  slow  and 
gradual,  while  here  beautiful,  symmetrical  forms  shape 
themselves  in  an  instant  out  of  this  liquid  mass,  reveal- 
ing to  us  an  architectural  power  in  what  we  call  lifeless 
matter,  whose  existence  and  controlling  influence  but 
few  of  us  have  probably  realized.  The  general  order 
of  the  phenomena  in  this  experiment  is  the  same  as  in 
the  last ;  but  notice  how  different  the  details.  We  do 
not  see  here  that  tendency  to  ramify  at  a  definite  angle, 
but  the  crystals  shoot  out  in  straight  lines,  and  cover  the 
plate  with  bundles  of  crystalline  fibres,  which  meet  or  in- 


CRYSTALLINE  STRUCTURE  OF  ICE.        53 

tersect  each  other  irregularly  as  the  accidental  directions 
of  the  several  shoots  may  determine  (Fig.  9).  As  before, 
we  cannot  recognize  the  separate  crystals  ;  indeed,  large 
isolated  crystals,  such  as  you  may  see  in  collections  of 
minerals,  cannot  be  formed  thus  rapidly.  They  are  of 
slow  growth,  and  only  found  where  the  conditions  have  fa- 
vored their  development.  But  all  the  mineral  substances, 
of  which  the  rocks  of  our  globe  consist,  have  a  crystal- 
line structure,  and  are  aggregates  of  minute  crystals  like 
the  arborescent  forms  whose  growth  you  have  witnessed. 
The  external  form  is  but  one  of  the  indications  of 
crystalline  structure,  and  by  various  means  this  structure 
may  frequently  be  made  manifest  when  the  body  appears 
wholly  amorphous.  Nothing  could  appear  externally 
more  devoid  of  structure  than  a  block  of  transparent 
ice.  Yet  it  has  a  most  beautiful  symmetrical  structure, 
which  can  easily  be  made  evident  by  a  very  simple  ex- 
periment, originally  devised,  I  believe,  by  Prof.  Tyn- 
dall.  For  this  purpose  I  have  prepared  a  plate  of  ice 
about  an  inch  in  thickness,  whose  polished  surfaces  are 
parallel  to  the  original  plane  of  freezing.  I  will  now 
place  this  plate  in  front  of  the  condenser  of  my  lantern, 
and,  placing  before  it  a  lens,  we  will  form  on  the  curtain 
an  image  of  the  ice-plate,  some  twenty  times  as  large  as 
the  plate  itself.  The  rays  of  heat  which  accompany 
the  light-rays  of  our  lantern  soon  begin  to  melt  the  ice ; 
but,  in  melting  it,  they  also  dissect  it,  and  reveal  its 
structure.  .  .  .  Notice  those  symmetrical  six-pointed 
stars  which  are  appearing  on  the  wall  (Fig.  10).  Prof, 
Tyndall  calls  them,  very  appropriately,  ice-flowers,  for, 
as  the  flower  shows  forth  the  structure  of  the  plant,  so 
these  hexagonal  forms  disclose  the  six-sided  structure 
of  ice.  You  can  hardly  fail  to  notice  the  similarity  of 
these  forms  to  those  of  the  snow-flake.  The  six  petals 


54 


THE   THREE   STATES  OF   MATTER. 


of  the  ice-flowers  on  onr  screen  make  with  each  other 
an  angle  of  60°,  and,  if  you  examine,  with  a  magnifier, 
flakes  of  fresh-fallen  snow  (Fig.  11),  or  the  arborescent 


FIG  10.— Ice-Flowers. 


forms  which  crystallize  on  the  window-panes  in  frosty 
weather,  you  will  find  that,  in  all  cases,  the  crystalline 
shoots  ramify  at  this  angle,  which  is  as  constant  a  char- 
acter of  the  solid  condition  of  water  as  is  the  right  an- 
gle of  sal-ammoniac. 

There  are  other  solids  whose  crystalline  structure, 
like  that  of  ice,  becomes  evident  during  melting  ;  but  a 
far  more  efficient  means  of  discovering  the  structure  of 
solids,  when  transparent,  is  furnished  by  polarized  light. 

It  would  be  impossible  for  me,  without  devoting  a 
great  deal  of  time  to  the  subject,  either  to  explain  the 
nature  of  wh:it  the  physicists  call  polarized  light,  or  to 
give  any  clear  idea  of  the  manner  in  which  it  brings 
out  the  structure  of  the  solid.  I  can  only  show  you  a 
few  experiments,  which  will  make  evident  that  such 
is  the  fact.  We  have  now  thrown  on  the  screen  a  lumi- 
nous disk,  which  is  illuminated  by  polarized  light.  To 
the  unaided  eye  it  does  not  appear  differently  from 


INDICATIONS   BY   POLARIZED   LIGHT. 


55 


ordinary  light;  but  there  is  this  peculiarity  in  the 
beam.  I  have  here  a  prism  of  well-known  construc- 
tion, made  of  Iceland-spar,  and  called  a  Nicol  prism. 


FIG.  11.— Snow-Crystals. 

The  spar  is  as  translucid  as  glass,  and,  with  ordinary 
light,  it  transmits,  as  you  see,  the  beam  equally  well, 
whether  it  is  placed  in  one  position  or  another.  But, 
with  the  polarized  beam,  we  shall  have  a  very  different 
result.  In  one  position,  as  you  notice,  it  allows  the 
light  to  pass  freely ;  but,  on  turning  it  round  through 
an  angle  of  90°,  almost  all  the  light  is  intercepted : 
the  beam  of  light  seems  to  have  sides,  which  stand  in 
a  different  relation  to  the  prism  in  one  position  from 
that  which  they  bear  to  it  in  the  other.  To  describe 
this  condition  of  the  beam,  the  early  experimenters 
adopted  the  word  polarized,  which  was  not,  however, 
a  happy  designation ;  for  the  term  now  implies  an 
opposition  of  relations  very  unlike  the  difference 
which  we  recognize  between  the  sides  of  such  a  beam 
of  light.  Placing  now  the  Nicol  prism  in  the  posi- 
tion in  which  it  intercepts  the  polarized  beam,  I  will 
first  place  between  it  and  the  source  of  light  a  plate 


56  THE   THREE   STATES   OF   MATTER. 

of  glass.  You  notice  that  there  is  no  difference  of 
effect.  Besides  the  arrangement  for  polarizing  the 
light  and  the  Nicol  prism  there  is  no  other  apparatus 
here  except  a  Jens,  which  would  form  on  the  screen 
an  image  of  the  glass  plate  or  of  any  thing  depicted 
upon  it,  were  it  not  for  the  circumstance  that  the 
Nicol  prism  cuts  off  the  light.  By  turning  the  Nicol 
so  that  the  polarized  light  can  pass,  and  putting  a 
glass  photograph  in  the  place  of  the  glass  plate,  you 
see  at  once  the  photograph  projected  on  the  screen. 
Having  turned  back  the  Nicol  until  the  light  is  again 
intercepted,  I  will  remove  the  photograph,  and  put 
in  its  place  a  thin  sheet  of  gypsum.  .  .  .  See  this 
brilliant  display  of  colors.  The  plate  of  gypsum  is  as 
colorless  and  transparent  as  the  glass,  and  the  gorgeous 
hues  result  from  the  decomposition  of  the  polarized 
light  produced  by  the  crystalline  structure  of  the 
gypsum.  I  will  next  turn  round  the  film  of  gypsum, 
and  you  notice  that  the  colors  gradually  fade  out  and 
finally  disappear.  As  we  turn  farther  they  reappear, 
and  so  on.  Evidently,  the  colors  are  only  produced  in 
a  definite  position  of  the  gypsum  plate  with  reference 
to  our  polarizing  apparatus.  Moreover,  as  I  can  readily 
show  you,  the  tint  of  color  depends  on  the  thickness 
of  the  film.  I  have  here  a  simple  geometrical  design 
formed  of  plates  of  gypsum  of  different  thicknesses,  and 
you  notice  that  each  plate  assumes  a  different  hue.  On 
turning,  however,  our  Nicol  prism  90°,  these  colors  are 
suddenly  exchanged  for  their  complementary  tints. 

It  is  obvious  that  any  colored  designs  might  be  re- 
produced in  this  way  by  combining  gypsum  plates  cut 
to  the  required  thickness  and  form,  as  in  mosaic  work ; 
and  I  will  now  show  you  a  number  of  beautiful  illus- 
trations of  this  peculiar  form  of  art.  .  .  .  But  you  can- 


EFFECTS  OF  GYPSUM  PLATES.  57 

not  appreciate  the  wonder  of  these  experiments  without 
bearing  in  mind  that  these  gypsum  mosaics  show  no 
color  whatever  in  ordinary  light,  consisting,  as  they  do, 
of  plates  which  appear  like  colorless  glass. 

Let  me  now  substitute  for  the  gypsum  designs  the 
glass  plate  on  which  we  recently  crystallized  urea,  and 
notice  that  the  crystals  of  this  substance,  which  we 
saw  form  on  the  glass,  yield  similar  brilliant  hues. 
The  experiment  becomes  still  more  striking,  if  we  crys- 
tallize the  salt  under  these  conditions.  I  will,  there- 
fore, take  another  glass  plate,  and,  having  smeared  it  as 
before  with  the  solution  of  urea,  I  will  place  it  in  the 
focus  of  my  lens  before  the  polarizer.  The  field  is  now 
perfectly  dark,  but,  as  soon  as  the  crystals  begin  to 
form,  you  see  these  colored  needles  shoot  out  on  the  dark 
ground,  presenting  a  phenomenon  of  wonderful  beauty. 

Now,  all  this  indicates  a  definite  structure,  and,  to 
those  familiar  with  these  phenomena,  they  point  to  a 
definite  conclusion  in  regard  to  this  structure.  I  wish 
I  could  fully  develop  the  argument  before  you,  but  this 
would  require  more  time  than  the  plan  of  my  lectures 
allows,  and  I  must  be  content  if  I  have  been  able  to 
impress  upon  your  minds  the  single  general  truth 
which  these  experiments  suggest.  You  saw  the  urea 
crystallize,  that  is,  assume  a  definite  structure,  and  you 
now  see  that  this  structure  so  modifies  the  polarized 
light  as  to  produce  these  gorgeous  hues.  You  have 
seen  similar  hues,  but  still  more  brilliant,  produced  by 
a  plate  of  gypsum,  and  I  can  only  add  that  the  conclu- 
sion which  the  analogy  suggests  is  legitimate,  and  sus- 
tained by  the  most  conclusive  evidence.  The  trans- 
parent plates  of  gypsum  have  as  definite  a  structure  as 
the  crystals  of  urea,  and  to  the  student  of  optics  these 
colors  reveal  that  structure  just  as  clearly  as  it  is  mani- 


58  THE  THREE  STATES  OF  MATTER. 

fested,  even  to  the  uninstructed  eye,  by  the  processes  of 
crystallization,  which  we  have  witnessed  this  evening. 

Would,  however,  that  I  could  convey  to  you  a  more 
definite  idea  of  the  nature  of  that  structure,  for  our 
theory  gives  us  a  very  clear  conception  of  what  we 
suppose  to  be  the  relations  of  the  molecules  in  these 
solid  bodies !  But  the  subject  is  a  difficult  one,  and  it 
would  require  a  long  time  to  make  the  matter  intelli- 
gible. Still,  by  the  aid  of  a  few  parallel  experiments, 
I  may  be  able  to  give  you,  at  least,  a  glimpse  of  the 
manner  in  which,  as  we  suppose,  the  structure  of  solid 
bodies  is  produced. 

Everybody  knows  that  a  magnetic  needle,  when 
free  to  move,  assumes  a  definite  position,  pointing,  in 
general,  north  and  south.  Now,  a  magnetic  needle  is  a 
needle  of  steel  (hardened  iron)  in  a  condition  which  we 
call  polarized,  and  the  great  globe,  on  which  we  live, 
is  in  a  similar  polarized  condition,  and  these  two  polar- 
ized bodies  assume  a  definite  position  toward  each 
other.  The  earth  and  the  needle  possess  magnetic  po- 
larity ;  but  there  are  other  modes  of  polarity,  and  what 
is  true  of  magnets  is  true  of  all  polarized  bodies  to  a 
greater  or  less  extent.  A  collection  of  polarized  bodies 
will  always  arrange  themselves  in  some  definite  position 
with  reference  to  each  other — will  form,  in  a  word,  a 
definite  structure.  Magnets  afford  the  simplest  means 
of  illustrating  this  principle ;  but  it  should  be  borne  in 
mind,  while  witnessing  the  experiments  I  am  to  show 
you,  that  the  truth  illustrated  has  a  very  wide  applica- 
tion. 

I  have  a  number  of  common  cambric  needles,  all 
magnetized  so  that  the  points  of  the  needles  are  their 
north  poles,  and  by  sticking  these  points  into  small 
corks  I  can  readily  make  the  needles  float  on  water 


MAYER'S  EXPERIMENT.  59 

in  a  vertical  position,  with  the  north  poles  all  upper- 
most. In  order  to  make  the  effects  visible  to  the  whole 
audience,  I  have  placed  a  small  tank  of  water,  having  a 
glass  bottom,  on  the  stage  of  my  vertical  lantern,  which 
is  so  arranged  that  by  means  of  a  combination  of  lenses 
and  mirrors  I  can  project  an  image  of  the  surface  of 
the  water  on  the  screen  before  you,  and  you  see  float- 
ing on  the  liquid  the  little  corks  which  hide  the  nee- 
dles hanging  below  them.  I  begin  with  three  of  these 
floating  magnets,  which  you  may  regard  as  representing 
polarized  molecules  of  matter.  Notice  now  that,  as  I 
bring  near  to  them  the  south  pole  of  a  bar-magnet,  the 
three  molecules  at  once  place  themselves  at  the  vertices 
of  an  equilateral  triangle.  Add  now  another  molecule, 
and  we  have  formed  a  square ;  a  fifth  gives  us  a  penta- 
gon ;  a  sixth  a  hexagon.  With  seven  molecules  we  may 
have  a  heptagon,  but  usually  the  little  corks  arrange 
themselves  in  a  hexagonal  pattern,  leaving  one  to  mark 
the  centre ;  and,  as  the  number  of  our  representative 
molecules  increases,  we  find  that  more  than  one  condi- 
tion of  equilibrium  becomes  possible.  Sometimes  the 
little  corks  arrange  themselves  around  two  others  which 
station  themselves  as  if  at  the  foci  of  an  ellipse.  But 
as  the  forms  become  more  complex,  the  equilibrium  be- 
comes less  stable,  and  we  cannot  readily  reproduce  such 
effects  under  the  conditions  we  have  here.  Any  one, 
however,  can  easily  repeat  this  experiment  with  a  bar- 
magnet,  a  few  needles  and  corks,  and  a  bowl  of  water, 
and,  when  perfect  steadiness  is  secured,  a  great  variety 
of  forms  may  be  produced.  The  experiment  was  de- 
vised by  Prof.  Mayer,  of  Hoboken,  and  it  furnishes  a 
most  striking  illustration  of  the  formative  power  of 
polar  forces.  As  these  little  magnets  marshal  them- 
selves in  a  definite  order,  it  seems  as  if  they  were  en- 


60  THE   THREE   STATES   OF  MATTER. 

dowed  with  intelligence,  but,  as  you  well  know,  the 
directive  force  comes  from  without,  not  from  within. 
The  magnetic  virtue  is  not  inherent  in  the  steel  nee- 
dles. It  has  been  induced  in  them  by  the  presence  of 
a  magnetized  body,  and  lasts  only  for  a  limited  time, 
after  the  inducing  cause  has  been  withdrawn.  Bodies 
of  soft  iron  become  magnetized,  on  the  approach  of  a 
magnet,  far  more  readily  than  those  of  steel ;  but  the 
polarity  disappears  as  soon  as  the  magnet  is  removed. 

The  power  of  a  magnet  to  magnetize  temporarily 
all  masses  of  iron  in  its  neighborhood  enables  us  to 
illustrate  the  formative  power  of  polar  forces,  in  a  man- 
ner which  shows  the  development  of  internal  struct- 
ure as  strikingly  as  did  the  last  experiment  the  pro- 
duction of  a  symmetrical  external  form.  If  we  bring  a 
bar-magnet  near  some  iron  filings  sprinkled  over  a 
plate  of  glass,  these  little  bits  of  iron  become  at  once 
polarized  by  induction  ;  and,  if  then  we  gently  tap  the 
glass,  the  iron  particles  will  swing  round  on  its  smooth 
surface,  and  arrange  themselves  in  the  most  wonderful 
way.  By  means  of  my  vertical  lantern  I  can  show  you 
this  effect  most  beautifully.  I  first  sprinkle  the  filings 
on  the  glass  stage  of  our  lantern,  and  then,  having  pro- 
tected them  by  a  thin  covering-glass,  I  bring  near  the 
glass  one  of  the  poles  of  a  bar-magnet.  .  .  .  Notice 
how,  on  tapping  the  glass,  the  filings  spring  into  posi- 
tion, arranging  themselves  on  lines  radiating  from  this 
pole  (Fig.  12).  Here,  evidently,  we  have  a  definite 
structure  produced.  Let  us  now  clear  our  stage,  and 
arrange  for  a  second  experiment.  This  time,  however, 
we  will  lay  the  bar-magnet  on  the  covering-glass,  so 
that  the  bits  of  iron  shall  be  brought  under  the  influence 
of  both  of  its  poles  at  the  same  time.  ...  See  what 
a  beautiful  set  of  curves  results  on  tapping  the  glass 


MAGNETIC   CURVES. 


61 


(Fig.  13),  and  let  me  beg  you  to  try  to  carry  in  your 
mind  for  a  moment  the  general  aspect  of  this  structure, 
as  well  as  of  the  first. 

Now,  we  suppose  that,  in  solid  bodies,  the  structure 


FIG.  12.— Magnetic  Curves,  one  pole.        FIG.  13.— Magnetic  Curves,  two  poles. 

depends  on  the  polarity  of  the  molecules,  and  that  the 
molecules,  like  the  bits  of  iron  in  our  experiment,  take 
up  the  relative  position  which  the  polar  forces  require. 
And,  next,  I  will  show  you  that  a  beam  of  polarized 
light  develops  in  some  solids  an  evidence  of  structure 
not  very  unlike  that  you  have  just  seen. 


Fio.  14.— Rings,  Uniaxial  Crystals. 


FIG.  15.— Eings,  Biaxial  Crystals. 


Keturning,  then,  to  our  polariscope,  I  place  in  the 
beam  of  light  a  plate  of  Iceland-spar  cut  in  a  definite 
manner.  .  .  .  See  those  radiating  lines,  and  those  iris- 
colored  circles  (Fig.  14).  Does  not  that  remind  you  of 
the  structure  we  developed  around  a  single  magnetic 
pole  ?  Next,  I  will  use  a  similar  plate  cut  from  a  crys- 


62  THE   THREE  STATES   OF  MATTER. 

tal  of  nitre ;  .  .  .  and,  see,  we  have  almost  the  repro- 
duction of  the  curves  about  the  double  pole  (Fig.  15).  It 
is  the  form  of  the  curves  as  indicating  a  certain  struct- 
ure, not  the  brilliant  colors,  to  which  I  would  direct  your 
attention.  The  iris  hues  are  caused  simply  by  the 
breaking  up  of  the  white  light  we  are  using ;  for  the 
crystal  decomposes  it  to  a  greater  or  less  extent,  like  a 
prism.  If,  by  interposing  a  plate  of  red  glass,  we  cut 
off  all  the  rays  except  those  of  this  one  color,  the  varied 
tints  disappear,  but,  in  the  black  curves  which  now  take 
their  place,  the  analogy  I  am  endeavoring  to  present 
becomes  still  more  marked.  Certainly,  you  could  have 
no  more  striking  analogy  than  this.  I  can  add  nothing 
by  way  of  commentary  to  the  experiments  without 
entering  into  unsuitable  details ;  but  I  will  say,  how- 
ever, that  I  am  persuaded  that  the  resemblances  we 
have  seen  have  a  profound  significance,  and  that  the 
structure,  which  the  polarized  beam  reveals  in  these 
solid  bodies,  is  really  analogous  to  that  which  the  mag- 
net produces  from  the  iron  filings. 

The  experiments  we  have  seen  conclusively  show 
that  an  external  form  and  an  internal  structure  re- 
sembling the  form  and  structure  of  crystalline  solids 
may  result  from  the  natural  grouping  of  small  masses 
of  iron  or  steel  polarized  by  the  presence  of  a  magnet. 
Of  course,  form  and  structure  imply  a  certain  amount 
of  tenacity ;  but  our  experimental  demonstration  will 
be  more  conclusive  if  I  further  show  that  not  only  te- 
nacity, but  even  a  strong  cohesive  force,  may  be  deter- 
mined between  otherwise  inert  masses  of  iron  by  mag- 
netic induction.  Here  is  a  powerful  electro- magnet, 
with  its  massive  horseshoe-shaped  core  of  soft  iron  and 
the  encircling  coils  of  insulated  copper  wire  which 
conduct  in  spiral  lines  around  the  core  the  electrical 


TENACITY,   HOW  INDUCED.  63 

current  that  renders  the  iron  strongly  magnetic;  and 
when  we  make  connection  with  our  dynamo-electrical 
machine,  notice  how  great  a  weight  the  magnet  will 
sustain.  But  notice,  also,  that  the  moment  the  elec- 
trical current  is  broken,  the  power  is  gone.  Covering 
now  the  poles  of  the  magnet  with  a  thin  board,  I  have 
a  table  on  which  I  can  pile  up  several  pounds  of  small 
wrought-iron  nails.  These  nails  are  perfectly  loose,  and 
have  no  more  attraction  for  each  other  than  grains  of 
sand.  Again  I  close  the  connection,  and  the  current 
flows  around  the  iron  core,  when  notice,  the  nails  be- 
come bound  into  a  compact  mass,  and  this  solely  in  con- 
sequence of  the  mere  presence  of  this  powerful  mag- 
net beneath  the  table.  Each  nail  has  become  magnet- 
ized by  induction,  and  the  resulting  mutual  attractions 
between  these  polarized  bits  of  iron  have  converted  the 
loose  particles  into  a  solid  body.  As  the  nails  can  slip 
to  a  limited  extent  on  each  other,  this  solid  mass  has  sin- 
gularly plastic  qualities ;  and,  as  you  see,  I  can  mould 
it  into  various  shapes.  .  .  .  Now,  we  have  a  Gothic  arch 
sustained  by  no  power  inherent  in  the  iron,  but  in  con- 
sequence of  a  power  induced  solely  by  the  presence  of 
the  electro-magnet.  We  break  the  current,  and  our 
arch  falls,  and  its  solid  walls  crumble  into  nails. 

Now,  I  could  bring  before  you  a  great  number  of 
facts  which  point  to  the  conclusion  that  the  cohesive 
force  which  holds  together  the  particles  of  this  crystal 
of  feldspar,  for  example,  is  a  polar  force  similar  to  mag- 
netism ;  and,  if  this  is  true  of  the  crystal  of  feldspar, 
it  must  be  true  of  all  solid  bodies.  Is,  then,  cohesion 
simply  a  manifestation  of  an  induced  polarity  ?  Is  there 
some  presence  in  Nature  which,  like  the  magnet  in  our 
experiments,  shapes  the  crystals,  adjusts  the  sprays  of 
the  snow-flake,  and  holds  the  mountains  in  its  grasp  ?  I 


64  THE  THREE   STATES  OF  MATTER. 

can  only  answer  that  the  analogies  I  have  brought  be- 
fore you  force  upon  my  mind  the  profound  conviction 
that  there  is.  Much  has  been  said  recently  about  the 
potency  of  matter.  I  can  find  no  evidence  of  any  po- 
tency inherent  in  matter.  As  I  conceive  of  them,  the 
ultimate  particles  of  matter  are  wholly  inert  and  passive, 
simple  magnitudes,  nothing  more.  But  everywhere  in 
Nature  there  seems  to  be  a  Presence  which  not  only 
imparts  power  to  these  particles,  but  also  directs  each 
particle  to  its  appointed  place.  We  are,  however,  trans- 
gressing the  legitimate  bounds  of  science.  This  specula- 
tion may  be  all  an  idle  fancy,  but  I  hope  that  the  study 
of  these  phenomena  of  magnetism  has  shown  at  least 
that  the  conception  is  not  absurd. 


LECTUKE  III. 

HOW  MOLECULES   ARE   WEIGHED. 

IN  order  that  we  may  make  sure  of  the  ground  we 
have  thus  far  explored,  let  me  recapitulate  the  charac- 
teristic qualities  of  the  three  conditions  of  matter  which 
I  sought  to  illustrate  in  the  last  lecture. 

A  gas  always  completely  fills  the  vessel  by  which  it 
is  inclosed.  It  is  in  a  state  of  permanent  tension,  and 
conforms  to  three  fundamental  laws — 

THE  LAW  OF  MAKIOTTE, 
THE  LAW  OF  CHAELES, 
THE  LAW  OF  AVOGADEO. 

The  first  two  are  independent  of  any  theory,  and  simply 
declare  that,  when  the  mass  is  constant,  the  volume  of 
every  gas  varies  inversely  as  the  pressure,  and  directly 
as  the  absolute  temperature ;  or,  if  the  volume  is  con- 
stant, that  the  mass  (or  weight)  varies  directly  as  the 
pressure,  and  inversely  as  the  absolute  temperature. 
The  third  law,  however,  is  based  on  the  molecular 
theory.  It  is  more  general,  and  includes  the  other  two. 
It  declares  that  equal  volumes  of  all  gases  under  the 


66  HOW   MOLECULES  ARE   WEIGHED. 

same  conditions  of  temperature  and  pressure  contain 
the  same  number  of  molecules. 

A  liquid  has  a  definite  surface.  It  can  be  only  very 
slightly  compressed,  and  obeys  neither  of  the  above  laws. 

A  solid  has  a  definite  structure,  and  resists  both 
longitudinal  and  shearing  stresses  to  a  greater  or  less 
extent. 

Having  now  presented  to  you  the  molecular  theory 
as  fully  as  I  can  without  entering  into  mathematical 
details,  I  come  back  again  to  the  great  law  of  Avoga- 
dro,  which  is  at  the  foundation  of  our  modern  chem- 
istry : 

When  in  the  condition  of  a  perfect  gas,  all  sub- 
stances',  under  like  conditions  of  temperature  and  press- 
ure, contain  in  equal  volumes  the  same  number  of  mole- 
cules. 

I  have  already  shown  you  that,  if  we  assume  the 
general  truth  of  the  molecular  theory  (in  other  words, 
if  we  assume  that  a  mass  of  gas  is  an  aggregate  of  iso- 
lated moving  molecules),  then  the  law  of  Avogadro 
follows  as  a  necessary  consequence  from  the  known 
properties  of  aeriform  matter,  and  may,  therefore,  in  a 
certain  limited  sense,  be  said  to  be  capable  of  proof. 
As  yet,  however,  we  have  only  considered  the  purely 
physical  evidence  in  favor  of  the  law.  But,  when  at 
the  next  lecture  we  come  to  study  the  chemical  evi- 
dence, we  shall  find  that  it  fully  sustains  the  conclusion 
which  has  been  deduced  from  our  molecular  theory  by 
the  principles  of  mechanics.  I  have  already  briefly 
referred  to  the  history  of  the  law. 

The  original  memoir  was  published  by  Amedeo 
Avogadro  in  the  Journal  de  Physique,  July,  1811.  In 
this  paper  the  Italian  physicist  "  enunciated  the  opinion 
that  gases  are  formed  of  material  particles,  sufficiently 


PROGRESS   OF  THE   INQUIRY.  67 

removed  from  one  another  to  be  free  from  all  recipro- 
cal attraction,  and  subject  only  to  the  repulsive  action 
of  heat ; "  and,  from  the  facts,  then  already  well  estab- 
lished, that  the  same  variations  of  temperature  and  press- 
ure produce  in  all  gases  nearly  the  same  changes  of  vol- 
ume, he  deduced  the  conclusion  that  equal  volumes  of 
all  gases,  compound  as  well  as  simple^  contain,  under 
like  conditions,  the  same  number  of  these  molecules. 

This  conception,  simple  and  exact  AS  it  now  appears, 
was  at  the  time  a  mere  hypothesis,  and  was  not  ad- 
vanced even  with  the  semblance  of  proof.  The  discov- 
ery of  Gay-Lussac,  that  gases  combine  in  very  simple 
proportions  by  volume,  was  made  shortly  after,  and, 
had  its  important  bearings  been  recognized  at  once,  it 
would  have  been  seen  to  be  a  most  remarkable  confir- 
mation of  Avogadro's  doctrine.  But  the  new  ideas 
passed  almost  unnoticed,  and  w^ere  reproduced  by  Am- 
pere in  1814,  who  based  his  theory  on  the  experiments 
of  Gay-Lussac,  and  defended  it  with  far  weightier  evi- 
dence than  his  predecessor.  Still,  even  after  it  was 
thus  reaffirmed,  the  theory  seems  to  have  received  but 
little  attention  either  from  the  physicists  or  the  chem- 
ists of  the  period.  The  reason  appears  to  have  been 
that  the  integrant  molecules  of  Avogadro  and  the  par- 
ticles of  Ampere  were  confused  with  the  atoms  of  Dai- 
ton,  and,  in  the  sense  wThich  the  chemists  of  the  old 
school  attached  to  the  word  atom,  the  proposition  ap- 
peared to  be  true  for  only  a  very  limited  number  even 
of  the  comparatively  few  aeriform  substances  which 
were  then  known.  Moreover,  the  atomic  theory  itself 
was  rejected  by  almost  all  the  German  chemists;  and, 
in  physics,  the  theory  of  a  material  caloric  then  pre- 
vailing was  not  enforced  by  the  new  doctrine.  In  a 
word,  this  beautiful  conception  of  Avogadro  and  Anr 


68  HOW   MOLECULES   ARE   WEIGHED. 

pere  came  before  science  was  ripe  enough  to  benefit 
by  it.  A  half-century,  however,  has  produced  an  im- 
mense change.  The  development  of  the  modern  the- 
ory of  chemistry  has  made  clear  the  distinction  between 
molecules  and  atoms,  while  the  number  of  substances 
known  in  their  aeriform  condition  has  been  vastly  in- 
creased. It  now  appears  that,  with  a  few  exceptions, 
all  these  substances  conform  to  the  law,  and  these  ex- 
ceptions can,  for  the  most  part  at  least,  be  satisfactorily 
explained.  On  the  other  side,  in  the  science  of  physics, 
more  exact  notions  of  the  principles  of  dynamics  have 
become  general,  and  the  dynamical  theory  of  heat 
necessarily  involves  the  law  of  equal  molecular  vol- 
umes. Thus,  this  theory  of  Avogadro  and  Ampere, 
which  remained  for  half  a  century  almost  barren,  has 
come  to  stand  at  the  diverging-point  of  two  great  sci- 
ences, and  is  sustained  by  the  concurrent  testimony  of 
both.  It  is  not,  then,  without  reason  that  we  take  this 
law  as  the  basis  of  the  modern  system  of  chemistry ; 
and,  starting  from  it,  let  us  see  to  what  it  leads : 

In  the  first  place,  then,  it  gives  us  the  means  of  de- 
termining directly  the  relative  weight  of  the  molecules 
of  all  such  substances  as  are  capable  of  existing  in  the 
aeriform  condition.  For,  it  is  obvious,  if  equal  volumes 
of  two  gases  contain  the  same  number  of  molecules ,  the  rel- 
ative weights  of  these  molecules  must  be  the  same  as  the 
relative  weights  of  the  equal  gas-volumes.  Thus,  a  cubic 
foot  of  oxygen  weighs  sixteen  times  as  much  as  a  cubic 
foot  of  hydrogen  under  the  same  conditions.  If,  then, 
there  are  in  the  cubic  foot  of  each  gas  the  same  number 
of  molecules,  each  molecule  of  oxygen  must  weigh  six- 
teen  times  as  much  as  each  molecule  of  hydrogen. 

It  is  much  more  convenient  in  all  chemical  calcula- 
tions to  use  the  French  system  of  weights  and  meas- 


FRENCH  SYSTEM   OF  WEIGHTS  AND  MEASURES.       69 

ures;  and  since,  through  modern  school-books,  the 
names  of  these  measures  have  become  quite  familiar 
to  almost  every  one,  I  think  I  can  refer  to  them  with- 
out confusion.  The  accompanying  table  will  serve  to 
refresh  your  memory,  and  may  be  useful  for  reference : 

The  metre  is  approximately  the  TO  , w o", OTO" ^^^^  °f  a 
quadrant  of  a  meridian  of  the  earth  measured  from  the 
pole  to  the  equator. 

TJie  metre  equals  10  decimetres  or  100  centimetres. 

The  cubic  metre,  or  stere,  equals  1,000  cubic  decime- 
tres or  litres. 

The  cubic  decimetre,  or  litre,  equals  1 ,000  cubic  cen- 
timetres. 

The  gramme  is  the  weight,  in  vacuo,  of  one  cubic 
centimetre  of  water  at  4°  centigrade  (the  point  of  maxi- 
mum density}. 

The  kilogramme  equals  1,000  grammes,  and  is,  there- 
fore, the  weight  of  one  cubic  decimetre  or  litre  of  water 
under  the  same  conditions. 

TJie  crith  is  the  weight,  in  vacuo,  of  one  litre  of 
hydrogen  gas  at  0°  centigrade  (the  freezing-point  of 
water),  and  at  76  centimetres  (the  normal  height  of  the 
barometer).  It  equals  0.09  of  a  gramme  very  nearly. 

The  metre  is  equal  to  3^  feet  nearly. 

TJie  litre  is  equal  to  If  pint  nearly. 

The  gramme  is  equal  to  15^  grains  nearly. 

The  kilogramme  is  equal  to  2^-  pounds  nearly. 

The  convenience  of  the  French  system  depends  not 
at  all  on  any  peculiar  virtue  in  the  metre  (the  standard 
of  length  on  which  the  system  is  based),  but  upon  the 
two  circumstances — 1 .  That  all  the  standards  are  divided 
decimally  so  as  to  harmonize  with  our  decimal  arithme- 
tic ;  and,  2.  That  the  measures  of  length,  volume,  and 
weight,  are  connected  by  such  simple  relations  that  any 


70  HOW   MOLECULES  ARE   WEIGHED. 

one  can  be  most  readily  reduced  to  either  of  the  other 
two.  In  order  to  make  clear  these  last  relations,  I  must 
ask  you  to  distinguish  between  two  terms  which  are 
constantly  confounded  in  the  ordinary  use  of  language, 
namely,  density  and  specific  gravity. 

The  density  of  a  substance  is  the  amount  of  matter 
in  a  unit-volume  of  the  substance.  In  the  English  sys- 
tem it  is  the  weight  in  grains  of  a  cubic  inch,  and  in  the 
French  system  the  weight  in  grammes  of  a  cubic  centi- 
metre. Thus  the  density  of  wrought-iron  is  1,966 
grains  English,  or  7.788  grammes  French.  So  also  the 
density  of  water  at  4°  centigrade  (the  point  of  maxi- 
mum density)  is  252.5  grains,  or  1  gramme. 

The  specific  gravity  of  a  substance  is  the  ratio  be- 
tween the  weight  of  the  substance  and  that  of  an  equal 
volume  of  some  other  substance  taken  as  a  standard. 
For  liquids  and  solids,  water  is  always  the  standard 
selected,  and  the  specific  gravity,  therefore,  expresses 
how  many  times  heavier  the  substance  is  than  water. 
It  can  evidently  be  found  by  dividing  the  density  of 
the  substance  by  the  density  of  water,  because,  as  we 
have  just  seen,  these  densities  are  the  weights  of  equal 
volumes.  Hence  the  specific  gravity  of  iron  equals — 

1966  grains         7.788  grammes  _  ^  ^ 
252.5  grains  °         1  gramme 

Of  course,  the  specific  gravity  of  a  substance  will  be 
expressed  by  the  same  number  in  all  systems ;  and,  fur- 
ther, in  the  French  system,  as  the  example  just  cited 
shows,  this  number  expresses  the  density  as  well  as  t)ne 
specific  gravity.  Density,  however,  is  a  weight,  while 
specific  gravity  is  a  ratio,  and  the  two  sets  of  numbers 
are  identical  in  the  French  system  only  because  in  that 
system  the  cubic  centimetre  of  water  has  been  selected 
as  the  unit  of  weight. 


SIMPLICITY  OF  THE  FRENCH    SYSTEM. 


71 


In  the  French  system,  then,  the  same  number  ex- 
presses both  the  specific  gravity  and  also  the  weight  of 
one  cubic  centimetre  of  the  substance  in  grammes ;  and, 
since  both  1,000  grammes  =  1  kilogramme,  and  1,000 
cubic  centimetres  =  1  litre,  it  expresses  also  the  weight 
of  one  litre  in  kilogrammes.  These  relations  are  shown 
in  the  following  table  : 

The  specific  gravity  of  a  liquid  or  solid  shows  how 
many  times  heavier  the  body  is  than  an  equal  volume 
of  water  at  4°  centigrade.  The  same  number  expresses 
also  the  weight  of  one  cubic  centimetre  of  the  substance 
in  grammes,  or  of  one  litre  in  kilogrammes. 


Alcohol. 


Water. 


Sulphur. 


Iron. 


Gold. 


Sp.  Gr.,    0.8  1. 

Density,  0.8  gram.       1.  gram. 

The  black  squares  are  supposed  to  represent  cubic 
centimetres.  If  assumed  to  represent  cubic  decimetres, 
then  the  weights  which  measure  the  densities  would  be 
in  kilogrammes  instead  of  grammes.  It  will  now  be 
seen  how  simple  it  is  in  the  French  system  to  calculate 
weight  from  volume.  "When  the  specific  gravity  of  a 
substance  is  given,  we  know  the  weight  both  of  one 
cubic  centimetre  and  of  one  litre  of  that  substance,  and 
we  have  only  to  multiply  this  weight  by  the  number 
of  cubic  centimetres,  or  of  litres,  to  find  the  weight  of 
the  given  volume.  Thus  the  weight  of  a  wrought-iron 
boiler-plate  \  centimetre  thick,  and  measuring  120  cen- 
timetres by  75,  would  be — 

0.5  x  120  x  75  x  7.788  =  35,046  grammes. 

In  general — 

W.=V.xSp.  Gr. 


72         HOW  MOLECULES  ARE  WEIGHED. 

When  Y.  is  given  in  cubic  centimetres,  the  resulting 
weight  will  be  in  grammes ;  when  in  litres,  the  weight 
will  be  kilogrammes. 

In  estimating  the  specific  gravity  of  gases,  we  avoid 
large  and  fractional  numbers,  by  selecting,  as  our  stand- 
ard, hydrogen  gas,  which  is  the  lightest  form  of  mat- 
ter known ;  but  we  thus  lose  the  advantage  gained  by 
having  the  unit-volume  of  our  standard  the  unit  of 
weight.  It  is  no  longer  true  that  W.=V.  xSp.  Gr. 
In  order  to  preserve  this  simple  relationship,  it  has 
been  found  convenient  to  use  in  chemistry,  for  estimat- 
ing the  weight  of  aeriform  substances,  another  unit 
called  the  crith.  The  crith  is  the  weight,  in  vacuo, 
of  one  litre  of  hydrogen  gas  at  0°  centigrade,  and 
with  a  tension  of  76  centimetres.  It  is  equal  to  0.09 
of  a  gramme  nearly.  We  may  now  define  the  density 
of  a  gas  as  the  weight  of  one  litre  of  the  substance  in 
criths,  and  its  specific  gravity  as  a  number  which  shows 
how  many  times  heavier  the  aeriform  substance  is  than 
an  equal  volume  of  hydrogen  under  the  same  condi- 
tions of  temperature  and  pressure.  We  always  esti- 
mate the  absolute  weight  of  a  gas  under  what  we  call 
the  standard  condition,  namely,  when  the  centigrade 
thermometer  marks  0°,  and  the  barometer  stands  at  76 
centimetres,  But,  in  determining  the  specific  gravity 
of  a  gas,  the  comparison  with  the  standard  gas  may  be 
made  at  any  temperature  or  pressure,  since,  as  all  gases 
are  affected  alike  by  equal  changes  in  these  conditions, 
the  relative  weights  of  equal  volumes  will  not  be  altered 
by  such  changes.  The  subject  may  be  made  more 
clear  by  the  following  table : 

The  specific  gravity  of  a  gas  shows  how  many  times 
heavier  the  aeriform  substance  is  than  an  equal  volume 
of  hydrogen  gas  under  the  same  conditions  of  tempera- 


DENSITY  AND   SPECIFIC   GRAVITY.  73 

ture  and  pressure.  The  same  number  also  expresses  the 
weight  in  criths  of  one  litre  of  the  gas  under  the  stand- 
ard conditions. 


Hydrogen.  Nitrogen.  Oxygen.  Chlorine. 


Sp.Gr.,      1  14  16  35.5 

Density,    1  crith.  14  criths.  16  criths.  35.5  criths. 

Now  we  have  again  W.  =~Vr.  x  Sp.  Gr.,  only  we 
must  remember  that  W.  here  stands  for  a  certain  num- 
ber of  criths,  V.  for  a  certain  number  of  litres,  and  Sp. 
Gr.  for  the  specific  gravity  of  the  gas  referred  to  hy- 
drogen, a  number  which  also  expresses  the  weight  of 
one  litre  of  the  gas  in  criths. 

To  return  now  to  the  subject  of  molecular  weights. 
If  one  litre  of  hydrogen  weighs  one  crith,  and  one  litre 
of  oxygen  sixteen  criths,  and  if  both  contain  the  same 
number  of  molecules,  then  each  molecule  of  oxygen 
must  weigh  sixteen  times  as  much  as  each  molecule  of 
hydrogen.  Or,  to  put  it  in  another  way,  represent  by 
n  the  constant  number  of  molecules,  some  billion  bill- 
ion, which  a  litre  of  each  and  every  gas  contains,  when 
under  the  standard  conditions  of  temperature  and 
pressure.  Then  the  weight  of  each  molecule  of  hydro- 
gen will  be  -  of  a  crith,  and  that  of  each  molecule  of 

oxygen  —  of  a  crith,  and  evidently 

1     16 

»—  :  —  =  l  :  16 
n      n 

that  is,  again,  the  weights  of  the  molecules  have  the 

same  relation  to  each  other  as  the  weights  of  the  equal 

7 


74  HOW  MOLECULES  ARE   WEIGHED. 

gas-volumes.  Excuse  such  an  obvious  demonstration, 
but  it  is  so  important  that  we  should  fully  grasp  this 
conception  that  I  could  not  safely  pass  it  by  with  a  few 
words.  It  is  so  constantly  the  case  that  the  simplest 
processes  of  arithmetical  reasoning  appear  obscure  when 
the  objects  with  which  they  deal  are  not  familiar. 

Since,  then,  a  molecule  of  any  gas  weighs  as  much 
more  than  a  molecule  of  hydrogen,  as  a  litre  of  the 
same  gas  weighs  more  than  a  litre  of  hydrogen,  it  is 
obvious  that,  if  we  should  select  the  hydrogen-molecule 
as  the  unit  of  molecular  weights,  then  the  number  rep- 
resenting the  specific  gravity  of  a  gas  would  also  ex- 
press the  weight  of  its  molecules  in  these  units.  For 
example,  the  specific  gravity  of  oxygen  gas  is  16,  that 
is.  a  litre  of  oxygen  is  sixteen  times  as  heavy  as  a  litre 
of  hydrogen.  This  being  the  case,  the  molecule  of 
oxygen  must  weigh  sixteen  times  as  much  as  the  mole- 
cule of  hydrogen,  and,  were  the  last  our  unit  of  molec- 
ular weights,  the  molecule  of  oxygen  gas  would  weigh 
16.  So  for  other  aeriform  substances.  In  every  case 
the  molecular  weight  would  be  represented  by  the 
same  number  as  the  specific  gravity  of  the  gas  referred 
to  hydrogen. 

Unfortunately,  however,  for  the  simplicity  of  our 
system,  but  for  reasons  which  will  soon  appear,  it  has 
been  decided  to  adopt  as  our  unit  of  molecular  weight 
not  the  whole  hydrogen -molecule,  but  the  half-mole- 
cule. Hence,  in  the  system  which  has  been  adopted, 
the  molecule  of  hydrogen  weighs  2  ;  the  molecule  of 
oxygen,  which  is  sixteen  times  heavier,  16  times  2,  or  32 ; 

the  molecule  of  nitrogen,  which  is  fourteen  times  heav- 

ft> 
ier,  14  times  2,  or  28  ;  and,  in  general,  the  weight  <§'  the 

molecule  of  any  gas  is  expressed  by  a  number  equal  to 
twice  its  specific  gravity  referred  to  hydrogen.  Noth- 


THE  UNIT   EMPLOYED.  75 

ing,  then,  can  be  simpler  than  the  finding  of  the  mo- 
lecular weight  of  a  gas  or  vapor  on  this  system.  We 
have  only  to  determine  the  specific  gravity  of  the  aeri- 
form substance  with  reference  to  hydrogen  gas,  and 
double  the  number  thus  obtained.  The  resulting  prod- 
uct is  the  molecular  weight  required  in  terms  of  the 
unit  adopted,  namely,  the  half-molecule  of  hydrogen. 
Perhaps  there  may  be  some  one  who,  having  lost  one 
or  more  of  the  steps  in  the  reasoning,  wishes  to  ask  the 
question,  Why  do  you  double  the  specific  gravity  in 
this  method  ?  Let  me  answer  by  recapitulating.  It  all 
depends  on  the  unit  of  molecular  weights  we  have  adopt- 
ed. Had  we  selected  the  whole  of  a  hydrogen-molecule 
as  our  unit,  then  the  number  expressing  the  specific  grav- 
ity of  a  gas  would  also  express  its  molecular  weight ; 
but,  on  account  of  certain  relations  of  our  subject,  not 
yet  explained,  which  make  the  half -molecule  a  more 
convenient  unit,  we  use  for  the  molecular  weights  a 
set  of  numbers  twice  as  large  as  they  would  be  on 
what  might  seem,  at  first  sight,  the  simpler  assumption. 
In  order  to  give  a  still  greater  definiteness  to  our 
conceptions,  I  propose  to  call  the  unit  of  molecular 
weight  we  have  adopted  a  microcrith,  even  at  the  risk 
of  coining  a  new  word.  We  already  have  become 
familiar  with  the  crith,  the  weight  of  one  litre  of  hy- 
drogen, and  I  have  now  to  ask  you  to  accept  another 
unit  of  weight,  the  half  hydrogen -molecule,  which  we 
will  call  for  the  future  a  microcrith.  Although  a  unit 
of  a  very  different  order  of  magnitude,  as  its  name  im- 
plies, the  microcrith  is  just  as  real  a  weight  as  the 
crith  or  the  gramme.  We  may  say,  then,  that 

A  molecule  of  hydrogen  weighs  2  microcriths. 
"  oxygen          "     32          « 

"  nitrogen        "     28          " 

"  chlorine         "     VI          " 


76  HOW   MOLECULES  ARE   WEIGHED. 

Now,  what  I  am  most  anxious  to  impress  upon 
your  minds  is  the  truth  that,  if  the  molecules,  as  we 
believe,  are  actual  pieces  of  matter,  these  weights  are 
real  magnitudes,  and  that  we  have  the  same  knowl- 
edge in  regard  to  them  that  we  have,  for  example,  in  re- 
gard to  the  weights  of  the  planets.  The  planets  are  visi- 
ble objects.  We  can  examine  them  with  the  telescope  ; 
and,  when  we  are  told  Jupiter  weighs  320  times  as 
much  as  the  earth,  the  knowledge  seems  more  real  to 
us  than  the  inference  that  the  oxygen-molecule  weighs 
32  microcriths.  But  you  must  remember  that  your 
knowledge  of  the  weight  of  Jupiter  depends  as  wholly 
on  the  law  of  gravitation  as  does  your  knowledge  of 
the  weight  of  the  molecules  of  oxygen  on  the  law  of 
Avogadro.  You  cannot,  directly,  weigh  either  the 
large  or  the  small  mass.  Your  knowledge  in  regard 
to  the  weight  is  in  both  cases  inferential,  and  the  only 
question  is  as  to  the  truth  of  the  general  principle  on 
which  your  inference  is  based.  This  truth  admitted, 
your  knowledge  in  the  one  case  is  just  as  real  as  it 
is  in  the  other.  Indeed,  there  is  a  striking  analogy 
between  the  two.  The  units  to  which  the  weights  are 
respectively  referred  are  equally  beyond  the  range  of 
our  experience  only  on  the  opposite  sides  of  the  com- 
mon scale  of  magnitude ;  for  what  more  definite  idea 
can  we  acquire  of  the  weight  of  the  earth  than  of  the 
molecule  of  hydrogen,  or  its  half,  the  microcrith  ?  It  is 
perfectly  true  that,  from  the  experiments  of  Maskelyne, 
Cavendish,  and  the  present  Astronomer-Royal  of  Eng- 
land, we  are  able  to  estimate  the  approximate  weight 
of  the  earth  in  pounds,  our  familiar  standard  of  weight ; 
and  so,  from  the  experiments  of  Sir  W.  Thompson,  we 
are  able  to  estimate  approximately  the  weight  of  the 
hydrogen -molecule,  and  hence  find  the  value  of  the 


MOLECULAR  WEIGHTS   REAL   MAGNITUDES.  77 

microcrith  in  fractions  of  the  crith  or  gramme.1  It  is 
true  that  the  limit  of  error  in  the  'last  case  is  very  much 
larger  than  in  the  first,  but  this  difference  is  one  which 
future  investigation  will  in  all  probability  remove. 

I  have  dwelt  thus  at  length  on  the  definition  of 
molecular  weight,  because,  without  a  clear  conception 
of  this  order  of  magnitudes,  we  cannot  hope  to  study 
the  philosophy  of  chemistry  with  success.  Our  the- 
ory, I  grant,  may  all  be  wrong,  and  there  may  be  no 
such  things  as  molecules  ;  but,  then,  the  philosophy  of 
every  science  assumes  similar  fundamental  principles, 
of  wrhich  the  only  proof  it  can  offer  is  a  certain  har- 
mony with  observed  facts.  So  it  is  with  our  science. 
The  new  chemistry  assumes  as  its  fundamental  pos- 
tulate that  the  magnitudes  we  call  molecules  are  reali- 
ties ;  but  this  is  the  only  postulate.  Grant  the  postu- 
late, and  you  will  find  that  all  the  rest  follows  as  a 
necessary  deduction.  Deny  it,  and  the  "  New  Chemis- 
try "  can  have  no  meaning  for  you,  and  it  is  not  worth 
your  while  to  pursue  the  subject  further.  If,  therefore, 
we  would  become  imbued  with  the  spirit  of  the  new 
philosophy  of  chemistry,  we  must  begin  by  believing 
in  molecules  ;  and,  if  I  have  succeeded  in  setting  forth 
in  a  clear  light  the  fundamental  truth  that  the  mole- 
cules of  chemistry  are  definite  masses  of  matter,  whose 
weight  can  be  accurately  determined,  our  time  has 
been  well  spent. 

Before  concluding  this  portion  of  my  subject,  it  only 
remains  for  me  to  illustrate  the  two  most  important 
practical  methods  by  which  the  molecular  weights  of 
substances  are  actually  determined.  It  is  evident  from 

1  According  to  Thompson,  one  cubic  inch  of  any  perfect  gas  contains, 
under  standard  conditions,  1C23  molecules.  Hence,  one  litre  contains 
61  x  1023  molecules  and  1  crith  =  122  x  1023  microcriths. 


78  HOW   MOLECULES  ARE  WEIGHED. 

what  has  been  said  that  we  can  easily  find  the  molecu- 
lar weight  of  any  substance  capable  of  existing  in  the 
state  of  gas  or  vapor,  by  simply  determining  experi- 
mentally the  specific  gravity  of  such  gas  or  vapor  with 
reference  to  hydrogen.  Twice  the  number  thus  ob- 
tained is  the  molecular  weight  required  in  microcriths. 

Now,  the  specific  gravity  of  an  aeriform  substance 
is  found  by  dividing  the  weight  of  a  measured  volume 
of  the  substance  by  the  weight  of  an  equal  volume  of 
hydrogen  gas  under  the  same  conditions.  This  simple 
calculation  implies,  of  course,  a  knowledge  of  two 
quantities  :  first,  the  weight  of  a  measured  volume  of 
the  substance,  and,  secondly,  the  weight  of  an  equal 
volume  of  hydrogen  gas  under  the  same  conditions. 
Of  these  two  weights,  the  last  can  always  be  calculated 
(by  the  laws  of  Mariotte  and  Charles)  from  the  weight 
which  a  cubic  decimetre  of  hydrogen,  under  the  stand- 
ard conditions,  is  known  to  have,  namely,  0.0896 
gramme  or  1  crith  ;  so  that  the  method  practically  re- 
solves itself  into  weighing  a  measured  volume  of  the 
gas  or  vapor  and  observing  the  temperature  and  press- 
ure of  the  substance  at  the  time.  There  are  always  at 
least  four  quantities  to  be  observed :  first,  the  volume  of 
the  gas  or  vapor  ;  secondly,  its  weight ;  thirdly,  its  tem- 
perature ;  fourthly,  its  tension  ;  and,  lastly,  the  weight 
of  an  equal  volume  of  hydrogen,  under  the  same  condi- 
tions, is  to  be  calculated  from  the  known  data  of  science. 

The  most  common  case  that  presents  itself  is  that 
of  a  substance  which,  though  liquid  or  even  solid  at 
the  ordinary  temperature  of  the  air,  can  be  readily 
converted  into  vapor  by  a  moderate  elevation  of  tem- 
perature; such  a  substance,  for  example,  as  alcohol. 
Now,  we  can  find  the  weight  of  a  measured  volume  of 
such  a  vapor  at  an  observed  temperature  and  tension 


DUMAS'  METHOD.  79 

in  one  of  two  ways,  both  of  which  are  in  general  use. 
In  the  first  process  we  fill  a  glass  globe  of  known  size 
with  the  vapor,  and  weigh  this  measured  volume.  In 
the  second,  we  weigh  out  in  a  liliputian  glass  bottle  a 
small  quantity  of  the  substance,  and,  having  converted 
the  whole  of  it  into  vapor,  we  measure  the  volume 
which  it  yields. 

The  first  process,  devised  by  Dumas,  of  Paris,  and 
known  by  his  name,  is  conducted  as  follows :  We  take 
a  glass  matrass  (a  thin  glass  globe,  with  a  long  neck), 
and,  heating  the  neck  in  a  glass-blower's  lamp  (as  near 
to  the  body  of  the  matrass  as  possible)  we  draw  it  out 
into  a  capillary  tube,  three  or  four  inches  long.  Hav- 
ing first  weighed  the  glass,  we  introduce  into  the  globe 
a  few  table- spoonfuls,  we  will  say,  of  pure  alcohol;  and 
this  we  can  readily  do  by  alternately  heating  and  cool- 
ing the  vessel.  We  then  mount  the  globe  in  a  brass 
frame,  and  sink  it  under  melted  paraffine,  but  so  that 
the  capillary  opening  shall  rise  above  the  surface  of 
the  hot  liquid.  A  common  iron  pot  serves  to  hold  the 
paraffine  (Fig.  18),  which  is  heated  over  a  gas-lamp, 
and  a  thermometer  dipping  in  the  bath  enables  us  to 
watch  the  temperature. 

Of  course,  the  alcohol  is  soon  volatilized,  and  the 
balloon  filled  with  its  vapor.  The  excess  escapes 
through  the  capillary  tube,  and,  by  lighting  the  jet,  we 
can  tell  when  the  vapor  in  the  globe  is  in  equilibrium 
with  the  external  air,  for  at  that  moment  the  flame 
will  go  out.  We  now,  with  a  blow-pipe,  melt  the 
glass  around  the  opening  of  the  capillary  tube,  and 
thus  hermetically  seal  up  the  vapor  in  the  globe.  At 
the  same  time  we  note  the  height  of  the  barometer 
and  the  temperature  of  the  bath.  The  height  of  the 
barometer  gives  us  the  tension  of  the  vapor  in  the  bal- 


80 


HOW    MOLECULES  ARE  WEIGHED. 


loon,  because,  at  the  moment  of  sealing,  the  tension 
was  equal  to  the  pressure  of  the  air  which  the  barome- 
ter directly  measures,  and  the  temperature  of  the  va- 
por must  be  the  same  as  the  temperature  of  the  bath. 


FIG.  18.— Dumas1  Method  of  finding  the  Specific  Gravity  of  Vapors. 

We  can  now  remove  the  globe,  and,  after  it  is 
cooled  and  carefully  cleaned,  weigh  it  at  our  leisure. 
We  must  remember,  however,  that  the  apparent  weight 
of  the  globe  in  the  balance  is  not  its  true  weight,  be- 
cause, like  a  balloon,  the  globe  is  buoyed  up  by  the  air 
it  displaces,  and  we  must  therefore  correct  the  ob- 
served weight  by  adding  to  it  the  weight  of  the  air 
displaced.  This  correction  our  knowledge  of  the  weight 
of  air  under  varying  conditions  enables  us  to  calculate 
with  the  greatest  accuracy,  assuming,  of  course,  that 
the  volume  of  the  globe  is  known ;  and,  when,  from 
the  weight  of  the  globe  thus  corrected,  we  subtract  the 
weight  of  the  glass  previously  found,  the  remainder  is 
the  weight  of  alcohol-vapor  which  just  filled  the  globe 
at  the  moment  of  sealing,  and  when  it  had  the  tem- 
perature and  pressure  we  have  noted. 

Of  the  four  quantities  required,  we  have  now  ob- 
served three,  namely,  the  weight  of  the  vapor,  its  tern- 


DUMAS'   METHOD.  81 

perature,  and  its  tension.  We  also  know  that  its  vol- 
ume was  that  of  the  globe  when  we  sealed  up  its 
mouth.  Since,  however,  we  use  a  new  globe  for  each 
determination,  we  have  always  to  measure  its  volume, 
and  this,  practically,  is  the  last  step  of  the  process. 
The  volume  is  most  readily  found  by  filling  the  globe 
with  water,  and  weighing.  The  weight  of  the  water 
in  grammes  gives  the  volume  of  the  globe  in  cubic 
centimetres  very  closely.  The  globe,  moreover,  is 
easily  filled,  because  the  condensation  of  the  vapor,  on 
cooling,  leaves  a  partial  vacuum  in  the  interior,  into 
which  the  water  rushes  with  great  violence  as  soon  as 
the  tip  is  broken  off  under  the  surface  of  the  liquid. 
Omitting  certain  small  corrections  which  it  is  not  best  to 
discuss  in  this  general  exposition  of  the  subject,  we  may, 
lastly,  arrange  our  calculation  in  the  following  form  : 

Determination  of  the  Molecular  weight  of  Alcohol,  by 
Dumas9  Method. 

Volume  of  glass  globe  .................     500  cubic  centimetres. 

Temperature  at  time  of  closing  .........     273°  centigrade. 

Height  of  barometer  measuring  the  ten-  )  76  centimetres. 

.....  )  -- 


sion  of  vapor  at  time  of  closing 


Weight  of  globe  and  vapor  .............     228.54  criths. 

Correction  for  buoyancy,  equal  to  weight  "] 
of  500  cubic  centimetres  of  air  at  0° 
cent,  and  76  centimetres,  the  tern-  j-     7.21      u 
perature  and  pressure  in  the  balance-  | 
case  when  the  globe  was  weighed.  .  .  J  - 

235.75     " 

"Weight  of  glass  .......................     230.          " 

"Weight  of  alcohol-vapor  ...............         5.75     " 

Weight  of  500  cubic  centimetres  of  hy-  " 

drogen  gas  at  273°,   and  76  c.   m.  u 

.found  by   calculation,  as   explained 
above  ............................ 

5.75  -f-  0.25=  23  sp.  gr.  of  alcohol-vapor. 
23  x  2  =  46  molecular  weight  of  alcohol. 


82  HOW  MOLECULES  ARE   WEIGHED. 

The  second  process  to  which  I  referred  was  origi- 
nally invented  by  Gay-Lussac,  but  recently  has  been 
very  greatly  improved  by  Professor  Hofmann,  of  Berlin. 
Hofmann's  apparatus  (Fig.  19)  consists  of  a  wide  barom- 


WT 

FIG.  19.— Hoftnann's  Method  of  finding  the  Specific  Gravity  of  Vapors. 

eter  -  tube,  about  a  metre  long,  and  graduated  into 
cubic  centimetres.  This  tube  is  filled  with  mercury, 
and  inverted  over  a  mercury-cistern,  as  in  the  experi- 
ment of  Torricelli  (Fig.  20).  The  mercury  sinks,  of 
course,  to  the  height  of  about  76  centimetres,  leaving 
a  vacuous  space  at  the  top  of  the  tube,  and  into  this 
space  is  passed  up  a  very  small  glass-stoppered  bottle, 
containing  a  few  criths  of  the  substance  to  be  experi- 
mented on.  Around  the  upper  part  of  the  tube  is  ad- 
justed a  somewhat  larger  tube,  also  of  glass,  which 
serves  as  a  jacket,  and  through  this  is  passed  steam 
(or  the  vapor  of  a  liquid  boiling  at  a  higher  tempera- 


HOFMANN'S  METHOD. 


83 


ture  than  water),  in  order  to  heat  the  apparatus  to  a 
constant  and  known  temperature. 

Let  us  suppose  that  the  substance,  whose  molecular 
weight  we  now  wish  to  find,  is  common  ether.     We 


FIG.  20.— Torricelli's  Experiment. 

begin  by  weighing  our  little  bottle,  first  when  empty, 
and  then  when  filled  with  ether,  thus  determining,  with 
great  accuracy,  the  weight  of  the  quantity  of  ether 
used.  With  a  little  dexterity  we  next  pass  the  bottle 
under  the  mercury  into  the  barometer-tube,  when  it 
at  once  rises  into  the  vacuous  space.  We  now  pass 
free  steam  through  the  jacket,  until  we  are  sure  that 
the  temperature  of  the  apparatus  is  constant  at,  say, 


84  HOW   MOLECULES  ARE  WEIGHED. 

100°  centigrade.  The  ether,  expanding  with  the  heat, 
soon  forces  out  the  glass  stopper  by  which  it  was  con- 
fined, and  evaporates  into  the  space  above  the  mercury, 
depressing  the  column.  At  first  the  column  oscil- 
lates violently,  but  it  soon  comes  to  rest,  and  we  can 
then  read  on  the  graduated  scale  the  volume  of  the 
vapor  which  the  weight  of  ether  taken  has  yielded. 
This  vapor  is  evidently  at  the  temperature  of  boiling 
water,  or  100°  centigrade  ;  but  what  is  its  tension  ? 

The  method  of  measuring  the  tension  will  be  ob- 
vious if  you  reflect  that,  in  this  apparatus,  the  press- 
tire  of  the  air  on  the  surface  of  the  mercury  in  the  cis- 
tern is  balanced  by  the  mercury  column  in  the  tube 
and  the  tension  of  the  vapor  pressing  on  the  upper 
surface  of  this  column.  Hence,  the  height  of  the  col- 
umn in  the  tube  will  be  less  than  that  of  a  true  barom- 
eter in  the  neighborhood  by  just  the  amount  of  this 
tension.  In  order  to  find  the  tension,  we  have,  there- 
fore, only  to  observe  the  height  of  the  barometer,  and 
subtract  from  this  the  height  of  the  column  in  our  tube, 
which  we  must  now  measure  with  as  much  accuracy  as 
possible.  Omitting,  as  in  the  previous  example,  a  few 
small  corrections,  our  calculation  will  now  appear  thus : 

Determination  of  the  Molecular  weight  of  Ether  by 
Gay-Lussac*  s  method,  improved  by  Hofmann. 

Weight  of  ether  taken 2.539  criths. 

Volume  of  vapor  formed 125  cubic  centimetres. 

Temperature  of  vapor •     100°  centigrade. 

Height  of  barometer 76  c.  m. 

Height  of  column  in  tube 19  c.  m. 

Tension  of  vapor 57  centimetres. 

Weight  of  125  cubic  centimetres  of  hy-  \ 

drogen  gas  at  100°  and  57  centime-  V  0.068G  of  a  crith. 

tres,  by  calculation ; 

2.539  -v-  0.0686  =  37  sp.  gr.  of  ether. 
37  x  2  =  74  molecular  weight  of  ether. 


LIMITATIONS  OF   OUR   METHODS.  85 

As  has  been  stated,  the  two  methods  of  determining 
molecular  weight,  just  described,  apply  only  to  those 
substances  which  can  be  readily  volatilized  by  a  moder- 
ate elevation  of  temperature.  With  some  slight  modi- 
fications, the  first  method  may  likewise  be  used  for  the 
permanent  gases ;  and,  by  employing  a  globe  of  porce- 
lain, the  late  St.-Claire  Deville  succeeded  in  determin- 
ing, in  the  same  way,  the  molecular  weight  of  several 
substances  which  do  not  volatilize  under  a  red  heat. 
More  recently  Victor  Meyer  has  devised  a  very  ingen- 
ious method l  of  determining  the  specific  gravity  of 
vapors,  which,  being  independent  of  the  temperature, 
can  be  used  at  the  highest  temperatures  that  the  required 
vessels  can  be  made  to  withstand ;  and  by  means  of  an 
apparatus  of  platinum,  heated  in  a  powerful  furnace, 
he  has  been  able  to  extend  very  considerably  our  knowl- 
edge in  the  same  direction.  But  a  great  number  of 
substances  cannot  be  volatilized  at  all  within  any  man- 
ageable limits  of  temperature,  and  a  still  larger  number 
are  so  readily  decomposed  by  heat  as  to  be  incapable 
of  existing  in  the  aeriform  condition.  The  molecular 
weight  of  such  bodies  cannot,  of  course,  be  determined 
by  direct  weighing.  In  most  cases,  however,  we  are 
able  to  infer  with  considerable  certainty  the  molecular 
weight  of  these  non-volatile  bodies  from  a  knowledge 
of  their  composition  and  other  chemical  relations ;  but, 
nevertheless,  there  are  numerous  instances  in  which  the 
conclusions  thus  drawn  are  very  questionable,  and  a 
great  deal  of  the  uncertainty,  which  still  obscures  the 
philosophy  of  our  science,  arises  from  this  circumstance* 

1  For  a  description  of  this  method  see  author's  "  Chemical  Philoso- 
phy," revised  edition,  1882,  page  35. 


LECTUKE  IY. 

LAW    OF   CONSERVATION   OF   MASS,    LAW   OF   DEFINITE   PRO- 
PORTIONS,   AND    LAW   OF    GAY-LUSSAC. 

ALL  the  processes  we  have  studied  thus  far  have  not 
involved  any  change  of  substance  in  the  materials  em- 
ployed. The  liquid  or  crystalline  films,  which  in  some 
of  our  experiments  produced  such  gorgeous  phenomena 
of  color,  were  not  altered  thereby.  The  bits  of  iron 
which  became  polarized  by  the  influence  of  magnetism 
still  remained  metallic  iron.  The  liquid  which  was 
crystallized  in  the  snow-flakes,  or  converted  into  steam 
in  our  glass  flask,  remained  the  same  familiar  substance 
water,  in  all  these  conditions.  Such  processes  as  these 
we  call,  in  general,  physical  processes  ;  and  all  modes 
of  motion,  and  all  mechanical  processes  of  the  arts  by 
which  various  materials  are  converted  into  useful  shapes 
without  altering  their  substance,  belong  to  this  category 

There  is  another  class  of  processes,  however,  anc 
even  a  larger  class,  whose  very  essence  consists  in  the 
change  of  one  or  more  substances  into  wholly  differen 
substances.    These  processes  are  frequently  accompanied 
by  striking  physical  phenomena,  such  as  the  develop 
ment  of  electricity,  heat,  or  light ;  but  the  essence 
the  process  is  always  a  change  of  substance.     Such  pro- 
cesses we  distinguish  as  chemical  processes,  and  we  speak 


FACTORS  AND   PRODUCTS.  87 

of  chemical  changes,  or  of  chemical  phenomena,  or  of 
chemical  qualities,  understanding  by  the  term  chemical 
such  processes,  such  changes,  such  phenomena,  or  such 
qualities,  as  cannot  be  manifested  without  a  change  of 
substance  ;  that  is,  a  change  so  fundamental  that  we  al- 
ways give  to  the  products  of  the  change  different  names 
from  those  of  the  materials,  or  factors,  with  which  the 
change  began. 

The  elements  of  every  chemical  change  are  these  : 
1.  One  or  more  substances  called  the  factor*]  with  which 
the  change  begins ;  2.  One  or  more  substances  called 
the  products,  with  which  the  change  ends.  As  just 
said,  the  chemical  change  may  be  accompanied  with 
the  manifestation  of  striking  physical  phenomena,  as 
the  burning  of  gunpowder,  with  the  rush  of  a  cannon- 
ball  ;  the  burning  of  coal,  with  the  development  of  heat ; 
or  the  solution  of  zinc  in  the  acid  of  a  voltaic  battery, 
with  the  flow  of  an  electrical  current :  but  these  are  not 
the  phenomena  which  it  is  the  special  province  of  the 
chemist  to  study — he  leaves  these  to  the  physicist ;  but, 
on  his  part,  he  inquires  in  every  case  what  are  the  fac- 
tors and  what  are  the  products  of  the  change  ;  and,  when 
a  new  process  is  discovered,  he  is  not  content  until  he 
can  clearly  point  out  all  the  substances  that  enter  into 
the  process,  as  well  as  all  the  substances  that  are  formed 
by  it. 

At  first  sight  chemical  processes  are  frequently  very 
obscure,  and  one  great  reason  is,  that  we  live  in  an  at- 
mosphere which  is  a  mixture  of  two  invisible  aeriform 
substances,  named  nitrogen  gas  and  oxygen  gas,  and 
these  substances,  especially  the  last,  are  constantly  en- 
tering as  factors  into  chemical  processes  without  our 
noticing  the  circumstance ;  and,  again,  the  products  of 
such  processes,  when  aeriform,  often  escape  notice  by 


88  CAUSE   OF   OBSCURITY. 

mingling  with  the  great  volume  of  the  air.  Now,  that 
we  are  on  our  guard,  we  are  seldom  deceived  by  the  in- 
tervention of  the  atmosphere ;  but  in  former  times,  when 
the  qualities  and  relations  of  aeriform  bodies  were  little 
known,  so  great  was  the  obscurity  thus  caused,  that  even 
the  familiar  processes  of  combustion  have  not  been  un- 
derstood until  within  a  century.  These  processes  of 
combustion  will  furnish  the  best  means  of  illustrating 
the  general  principles  and  facts  we  have  just  stated. 

The  burning  of  a  log  of  wood  is  a  chemical  process, 
because,  in  burning,  the  material  we  call  wood,  together 
with  a  quantity  of  oxygen  gas  from  the  atmosphere 
disappear,  and  other  substances,  which  in  the  aggre- 
gate we  call  smoke  and  ashes,  appear  in  their  place. 
Our  fathers  overlooked  wholly  the  oxygen  gas,  and 
made  a  small  account  of  the  smoke,  and  it  is  no  won- 
der that  they  misunderstood  the  process ;  but  now  we 
know  all  the  factors  and  all  the  products.  Here  both 
the  factors  and  the  products  are  complex,  for  wood  is 
an  organism  containing  other  substances  than  wood- 
fibre,  and  smoke,  although  consisting  chiefly  of  aqueous 
vapor  and  carbonic  dioxide  gas,  carries  various  empyreu- 
matic  products.  A  burning  candle,  especially  if  it  con- 
sists of  some  definite  substance,  will  give  a  more  apt 
illustration.  Here  the  products  are  two  definite  sub- 
stances, oxygen  gas  and  the  material  of  the  candle  ;  and 
the  products  two  equally  definite  substances,  aqueous 
vapor  and  carbonic  dioxide.  That  these  products  are 
actually  escaping  from  this  candle-flame,  I  can  easily 
show  you.  If  I  hold  a  cold  glass  bell  over  the  flame, 
the  inner  surface  soon  becomes  bedewed,  and  after  a 
while  drops  of  liquid  water  run  down  the  sides.  Car- 
bonic-dioxide gas,  or  carbonic  acid,  as  it  is  often  called, 
may  not  be  to  every  one  as  familiar  a  substance  as  water, 


EXTENT  OF  OUR  KNOWLEDGE.  89 

but  it  is  equally  common.  It  is  the  gas  which  escapes 
from  all  effervescing  drinks,  and  it  has  one  characteris- 
tic property :  it  immediately  renders  a  solution  of  lime 
(lime-water)  turbid  if  brought  in  contact  with  the  liquid. 
I  uncork  some  bottled  beer  and  pour  it  into  a  tall  glass. 
As  the  effervescence  subsides,  a  colorless  gas  collects  in 
the  upper  part  of  the  glass ;  for  the  gas  is  so  heavy  that 
it  only  quite  slowly  diffuses  into  the  atmosphere.  This 
heavy  gas  is  carbonic  dioxide,  the  substance  of  which 
I  have  been  speaking.  Into  another  tall  glass  I  pour 
some  clear  lime-water,  and  now  fill  up  the  glass  with 
carbonic  dioxide,  which  I  can  readily  pour  off  from  the 
top  of  the  beer ;  and  notice  that,  when  I  shake  up  the 
carbonic  dioxide  gas  with  the  lime-water,  the  last  be- 
comes very  turbid,  owing,  in  fact,  to  the  formation  of 
chalk.  Now,  let  us  test  the  products  of  the  burning 
candle  which  we  have  been  collecting  in  the  glass 
bell.  I  invert  the  bell,  pour  into  it  some  of  the  same 
clear  lime-water,  which  at  once  becomes  turbid  as  be- 
fore. 

"What  is  true  of  the  chemical  process  we  have  just 
studied  is  true  of  all  chemical  processes.  There  are 
always  one  or  more  factors  and  one  or  more  products, 
and  it  is  one  of  the  great  objects  of  chemical  investi- 
gation to  find  out  what  are  these  factors  and  what  are 
these  products.  Moreover,  so  great  have  been  the  ad- 
vances in  chemical  knowledge  during  the  last  century 
that  we  actually  do  know  what  are  the  factors  and  what 
are  the  products  in  almost  all  the  chemical  processes 
which  occur  in  Nature  or  can  be  produced  by  art.  Fur- 
thermore, we  have  discovered  two  all-important  and 
fundamental  laws  which  govern  chemical  changes,  and 
to  these  I  wish  to  direct  your  special  attention. 

The  first  of  those  laws  appears  in  the  fact,  univer- 


90  CONSERVATION  OF  MASS. 

sally  observed,  that  in  every  chemical  reaction  the  sum 
of  the  weights  of  the  products  is  exactly  equal  to  the 
sum  of  the  weights  of  the  factors.  Thus,  with  the  burn- 
ing candle  the  weight  of  the  carbonic  dioxide  and 
water  formed  is  exactly  equal  to  the  weight  of  the 
material  of  the  candle  and  of  the  oxygen  gas  consumed. 
Although  in  this  familiar  experiment  it  is  easy  to  deter- 
mine the  weight  of  the  carbonic  acid  and  water  formed, 
and  also  the  weight  of  the  material  of  the  candle  burned, 
it  would  be  impracticable  to  weigh  directly  the  quan- 
tity of  oxygen  withdrawn  from  the  atmosphere  in  the 
process ;  and,  in  order  to  illustrate  the  great  law  under 
consideration,  we  must  turn  to  a  simpler  although  a  less 
familiar  experiment. 

In  this  glass  bulb  I  have  a  known  weight  of  a  black 
powder  called  oxide  of  copper.  Connected  with  it  on 
one  side  is  a  gasometer  containing  hydrogen  gas,  and 
so  arranged  that  I  can  measure  the  exact  volume  of 
gas  delivered.  On  the  other  side  is  connected  an  ab- 
sorption-tube which  will  retain  all  the  water  that  passes 
into  it ;  and,  finally,  the  absorption-tube  leads  into  a 
second  gasometer,  which  will  hold  and  enable  me  to 
measure  the  volume  of  hydrogen  which  escapes  uncon- 
sumed.  The  experiment  consists  simply  in  this :  We 
gently  heat  the  glass  bulb  containing  the  oxide  of  cop- 
per with  the  flame  of  a  lamp,  and  then  slowly  pass  a 
stream  of  hydrogen  gas  through  the  apparatus  from 
one  gasometer  to  the  other.  We  soon  see  that  out  of 
the  black  powder  a  red  metal  is  formed  in  the  bulb, 
which  we  recognize  instantly  as  copper ;  we  see  also 
that  vapor  of  water  passes  over  into  the  absorption-tube 
where  it  is  all  retained  ;  and  we  further  find,  after  the 
most  searching  examination,  that  water  is  the  only  other 
product..  The  chemical  process  may  be  stated,  very 


WEIGHT  AND  MASS.  91 

simply,  thus :  oxide  of  copper  and  hydrogen  gas  yield 
metallic  copper  and  water. 

Further,  the  parts  of  the  apparatus  are  so  arranged 
that  they  can  be  taken  apart  and  accurately  weighed 
both  before  and  after  the  experiment,  and  we  thus 
learn  the  weight  both  of  the  metallic  copper  and  of  the 
water  formed,  and  also  the  weight  of  the  oxide  of  cop- 
per used.  The  weight  of  the  hydrogen  gas  consumed 
we  can  calculate,  knowing  the  volume  that  has  disap- 
peared, by  a  comparison  of  the  two  gasometers,  and  we 
then  shall  find  that — 

Weight  of  Factors.  Weight  of  Products. 

Hydrogen  Gas  and  Oxide  of  Copper        =       Metallic  Copper  and  Water. 

and  this  is  simply  an  illustration  of  a  universal  truth 
which  all  chemical  investigation  confirms. 

In  discussing  this  subject  we  must  be  very  careful 
not  to  confound  the  two  uses  of  the  word  weight.  An 
ounce  or  a  pound  weight  may  be  either  a  measure  of 
force  or  a  measure  of  quantity  of  material.  When  we 
use  a  "  hundred- weight "  with  cord  and  pulley  to  meas- 
ure the  strength  of  a  man's  muscle,  we  simply  balance 
his  muscular  force  against  the  force  of  gravitation,  which 
draws  that  mass  of  iron  toward  the  centre  of  the  earth, 
and  this  force  varies  to  a  limited  extent  as  we  move 
over  the  surface,  slightly  increasing  as  we  travel  from 
the  equator  toward  the  pole ;  and  we  know  that  if  we 
could  carry  the  same  mass  of  iron  to  the  surface  of 
other  planets  we  should  find  that,  while  on  the  moon 
the  strength  of  an  infant's  arm  would  be  adequate  to 
lift  it,  on  Jupiter  the  power  of  a  Hercules  would  be 
insufficient  to  stir  it  from  the  ground.  Such  a  weight 
is  a  fixed  measure  of  force  only  so  far  as  the  force  of 
gravitation  is  invariable. 

In  chemistry  we  seldom  have  occasion  to  use  weights 


92  CONSERVATION  OF  MASS. 

as  a  measure  of  force,  and,  when  we  speak  of  a  certain 
weight  of  material,  we  refer  to  a  definite  quantity  of 
that  material.  We  weigh  out  one  hundred  pounds  of 
sugar  by  placing  the  "hundred-weight"  in  one  pan  of 
a  balance  and  adding  sugar  to  the  other  pan,  until  the 
point  of  equilibrium  is  reached,  and  we  then  have  an 
invariable  quantity  of  sugar,  the  same  in  all  places  and 
under  all  conditions ;  for,  however  much  the  force  of 
gravitation  may  vary,  the  effect  on  the  sugar  would  be 
the  same  as  the  effect  on  the  weights,  and  the  equi- 
librium would  not  be  disturbed.  Moreover,  since  the 
amount  of  material  is  exactly  proportional  to  its  weight 
thus  estimated,  fifty  pounds  of  sugar  being  exactly  one 
half  as  much  material  as  one  hundred  pounds  of  sugar, 
weight  becomes  the  measure  of  material  without  any 
reference  whatever  to  the  force  of  gravitation,  which 
is,  as  it  were,  the  medium  of  the  measurement.  When 
we  speak  of  a  certain  weight  of  material,  whether  it  be 
sugar,  coffee,  or  iron  nails,  we  convey  the  idea  simply 
of  a  certain  quantity  of  material  and  nothing  more ;  and 
in  order  to  avoid  the  confusion  which  is  apt  to  arise 
from  the  double  meaning  of  the  word  weight,  it  is  cus- 
tomary in  the  science  of  physics,  when  we  wish  to  desig- 
nate the  amount  of  material  in  a  body,  to  use  the  word 
mass.  In  the  experiment  we  have  just  described  we 
should  say  that  the  mass  of  the  oxide  of  copper  and 
hydrogen  gas  together  was  exactly  equal  to  the  united 
masses  of  the  copper  and  the  water,  or,  in  general,  that 
in  every  chemical  reaction  the  mass  of  the  products 
was  equal  to  the  mass  of  the  factors.  On  a  former 
occasion  I  have  spoken  of  this  general  truth  as  the  LAW 
OF  THE  CONSERVATION  OF  MASS.  The  fitness  of  the  ex- 
pression is  obvious,  and  it  appears  to  have  been  very 
generally  accepted. 


FAMILIAR   ILLUSTRATION.  93 

The  law  of  conservation  of  mass  in  chemical  pro- 
cesses is  the  extension  of  a  principle  which  is  so  obvious, 
in  all  processes  where  there  is  no  change  of  substance, 
that  it  almost  seems  self-evident.  When  an  ingot  of 
gold  is  coined,  the  amount  of  metal  in  the  coins  is  pre- 
cisely the  same  as  that  which  was  formerly  in  the  bar. 
Moreover,  the  material  of  the  coins  is  actually  the  same 
identical  material  as  was  formerly  in  the  ingot.  Is  it, 
then,  also  true  that  the  materials  of  the  water  and  copper 
which  were  the  products  of  the  chemical  process  just 
studied,  are  also  actually  the  same  as  the  materials  of 
the  oxide  of  copper  and  hydrogen  gas  from  which  those 
products  were  formed  ?  The  most  obvious  inference 
would  be  that  the  products  are  in  fact  formed  of  the 
same  material  as  the  factors,  and  that  mass  is  an  attri- 
bute of  matter  underlying  those  accidents  in  which  sub- 
stances differ ;  but  we  must  be  careful  to  distinguish  this 
inference  from  the  great  law  of  conservation  of  mass, 
which  is  an  established  fact  of  Nature. 

In  all  chemical  processes  it  is  not  only  true  that  the 
sum  of  the  weights  of  the  products  is  equal  to  the  sum 
of  the  weights  of  the  factors,  but  it  is  also  true  that 
the  weights  of  the  several  products  and  factors  stand  in 
a  definite  relation  to  each  other.  Thus,  in  our  previ- 
ous illustration,  it  is  not  only  true  that  the  weight  of 
the  oxide  of  copper,  plus  the  weight  of  the  hydrogen 
gas,  equals  the  weight  of  the  copper,  plus  the  weight  of 
the  water,  but  it  is  further  true  that — 

Weight  of       .      Weight  of     .  Weight  of  .  Weight  of  _  hrn  o  .  o  .  *o  o  .  -i  o 
Oxide  of  Copper  •  Hydrogen  Gas  •      Copper  •      Water       -  iy'6  '  "  •  °^'rf  •  u 

and  this  relation  is  invariable.  This  is  a  single  exam- 
ple of  a  general  principle  which  is  called  the  LAW  OF 
DEFINITE  PROPORTIONS.  We  might  multiply  such  ex- 
amples to  an  unlimited  extent,  for  they  are  as  numer 


94  DEFINITE   PROPORTIONS. 

oiis  as  are  the  known  definite  chemical  processes.  This, 
however,  is  unnecessary  at  this  time ;  for  not  only  is  the 
example  cited  a  very  apt  illustration  of  the  principle 
under  discussion,  but,  moreover,  we  shall  repeatedly 
meet  with  similar  examples  as  we  proceed  in  our  course. 
Furthermore,  as  we  shall  find,  the  science  of  chemistry 
enables  us  to  predict  what  in  any  case  these  proportions 
will  be,  so  that  the  law  of  definite  proportions  will  come 
to  appear  as  self-evident  as  the  law  of  conservation  of 
mass.  But  this  is  to  anticipate,  and  it  is  sufficient  for 
the  present  if  we  have  fully  grasped  the  great  funda- 
mental conception  of  chemistry  which  we  have  named 

THE  LAW  OF  DEFINITE  PROPORTIONS. 

In  order  to  complete  this  portion  of  my  subject,  I 
must,  before  closing  my  lecture,  ask  your  attention  to 
another  general  principle,  as  fundamental  and  as  fully 
based  on  observed  facts  as  the  two  we  have  already 
studied.  The  general  truth  to  which  I  refer  may  be 
stated  thus:  Whenever  in  a  chemical  process  two  or 
more  of  the  factors  or  products  are  either  aeriform, 
or  capable  of  existing  in  the  state  of  vapor,  we  always 
find  that  the  definite  proportions  observed  in  the  chemi- 
cal process  are  either  the  proportions  of  the  vapor  or  gas 
densities,  or  else  some  simple  multiple  of  these  propor- 
tions. Thus,  in  the  experiment  to  which  we  have  so 
frequently  referred  in  this  lecture,  the  hydrogen  gas 
and  water  which  fulfill  the  necessary  conditions  have 
the  relative  gas  or  vapor  densities  of  1 : 9,  and  you  notice 
that— 

Weieht  of  Weight  of  Density  of  Density  of 

Hydrogen  Water  Hydrogen  Steam 

2  :  18         =  1  :  9, 

and  so  in  all  cases.     A  different  phase  of  this  law  was 
first  observed  by  Gay-Lussac,  but,  as  he  saw  the  truth 


IMPORTANT   DEDUCTIONS.  95 

from  a  somewhat  different  point  of  view,  he  expressed 
it  in  a  different  way.  Still,  the  general  principle  in- 
volved in  the  two  statements  is  the  same,  and  we  may, 
therefore,  designate  this  third  law  as  the  law  of  Gay- 
Lussac.  Pass  now  to  the  inference  which,  after  what 
we  have  learned,  the  general  truth  just  stated  suggests. 
As  we  have  seen,  the  ratio  of  the  gas  or  vapor  densities 
of  any  substances  always  stands  in  the  direct  ratio  of 
their  molecular  weights.  Hence,  it  follows  that  the 
definite  proportions  of  which  we  have  been  speaking 
are  always  the  proportions  of  the  molecular  weights  of 
the  substances  involved  in  the  chemical  process  in  ques- 
tion, or  else  some  simple  multiples  of  these  proportions. 
In  the  case  we  have  cited,  the  ratio  of  2 : 18  is  the  ratio 
of  the  molecular  weight  of  hydrogen  gas  to  the  mo- 
lecular weight  of  water.  In  other  cases  we  should  find 
that  the  definite  proportion  observed  in  the  chemical 
process  would  be  the  ratio  of  the  molecular  weight  of 
one  substance  to  twice  or  thrice  the  molecular  weight 
of  another,  and  sometimes  of  twice  the  molecular  weight 
of  one  substance  to  thrice  the  molecular  weight  of  an- 
other ;  but  the  proportions  are  seldom  more  complex 
than  these. 

Finally,  there  are  three  important  deductions  which 
immediately  flow  from  the  principles  we  have  dis- 
cussed : 

In  the  first  place,  it  will  be  obvious  how  very  greatly 
these  chemical  facts  confirm  the  molecular  theory.  Thus 
far  we  have  based  this  theory  on  physical  phenomena 
alone,  and  we  have  deduced  the  molecular  weights  of 
substances  from  the  densities  of  these  substances  when 
in  the  condition  of  gas  or  vapor.  ISTow,  we  find  these 
same  values  reappearing  in  purely  chemical  phenomena, 
and,  if  there  are  such  things  as  molecules,  we  should 


96  LAW  OF  GAY-LUSSAC. 

naturally  expect  that  in  a  chemical  process  the  action 
would  take  place  between  the  molecules  of  the  sub- 
stances involved,  and,  if  so,  the  definite  proportions  ob- 
served must  be  some  multiples  of  the  relative  molecular 
weights,  as  we  find  that  they  are. 

In  the  second  place,  it  can  easily  be  seen  that  the 
definite  proportions  observed  in  chemical  processes  may 
give  the  means  of  correcting  the  molecular  weights 
deduced  from  determinations  of  gas  or  vapor  density. 
Such  determinations  can  rarely  be  made  with  accuracy, 
and  there  are  known  to  be  causes  independent  of  the 
molecular  weight  which  influence  the  density  of  aeri- 
form substances  to  a  limited  extent.  The  definite  pro- 
portions, on  the  other  hand,  can  usually  be  determined 
with  great  accuracy,  and  are  invariable.  It  is  true  that 
in  a  new  problem  we  may  not  be  able  to  tell  whether 
the  proportion  is  the  ratio  of  the  weights  of  single 
molecules,  or  of  several  molecules ;  but  it  gives  us  an 
exact  ratio,  and,  by  comparing  this  with  the  approxi- 
mate ratio  of  the  weights  of  single  molecules  obtained 
from  the  gas  or  vapor  densities,  we  can  at  once  inter- 
pret the  result,  and  deduce  in  each  case  the  correct 
value  of  the  molecular  weight  sought.  Or,  in  other 
words,  the  gas  or  vapor  densities  give  us  an  approxi- 
mate value  of  the  ratio  between  the  weights  of  single 
molecules.  The  definite  proportions  give  us  the  ex- 
act value  of  the  ratio  between  the  weight  of  single  or 
multiple  molecules,  as  the  case  may  be.  By  comparing 
the  two  we  can  see  at  a  glance  for  which  of  the  possible 
multiples  the  definite  proportions  stand,  and  we  can 
then  very  easily  deduce  an  accurate  value  of  the  simple 
ratio  at  first  only  approximately  known. 

In  the  third  place,  the  definite  proportions  observed 
in  chemical  processes  enable  us  to  determine  with  cer- 


IMPORTANT   DEDUCTIONS.  97 

tain  limitations  the  molecular  weights  of  non-volatile 
substances,  to  which  the  vapor-density  methods  are  ob- 
viously inapplicable.  Thus,  in  the  process  described  on 
page  90  neither  the  oxide  of  copper  used  nor  the  metal- 
lic copper  formed,  is  a  substance  whose  vapor-density 
can  be  determined.  But,  in  the  proportion  already 
given — 

Weight  of  Weight  of  Weight  of  Weight  of 

Oxide  of  Copper,  Hydrogen  Gas,  Copper,  Water, 

79.3  :  2  :  63.3  :  18 

if  two  microcriths  is  the  weight  of  a  molecule  of  hydro- 
gen gas,  then  79.3  must  be  the  weight  of  a  molecule  of 
oxide  of  copper,  and  63.3  the  weight  of  a  molecule  of 
copper,  or  else  these  two  values  are  multiples  of  the 
molecular  weights ;  and  with  this  limitation  we  can  thus 
determine  the  molecular  weights  of  all  similarly  non- 
volatile substances.  Moreover,  in  most  instances,  prin- 
ciples or  analogies  of  chemistry,  of  which  we  shall  gain 
some  knowledge  as  we  proceed,  enable  us  to  decide 
whether  we  are  dealing  with  multiple  molecules  or  not. 
There  are,  however,  many  cases  in  which  these  guides 
are  insufficient,  and  then  our  knowledge  is  uncertain  to 
just  that  extent.  But  we  have  now  pushed  this  discus- 
sion as  far  as  can  be  profitable  at  this  time.  Indeed,  I 
fear  that  you  have  found  it  abstruse  and  dull.  But  in 
chemistry,  as  in  other  sciences,  we  must  apprehend  the 
fundamental  conceptions  before  we  can  advance  in  our 
study,  and  you  will  not  regret  the  tedium  it  may  have 
involved,  if  you  gain  a  clear  conception  of  the  three 
great  laws  on  which  the  whole  superstructure  of  chem- 
istry rests — 

THE  LAW  OF  CONSERVATION  OF  MASS, 
THE  LAW  OF  DEFINITE  PROPORTION, 
THE  LAW  OF  GAY-LUSSAC. 


LECTUEE  V. 

CHEMICAL     COMPOSITION — ANALYSIS    AND    SYNTHESIS — THE 
ATOMIC   THEORY. 

IN  my  previous  lectures  I  have  endeavored  to  give 
you  a  clear  idea  of  the  meaning  which  our  modern 
science  attaches  to  the  word  molecule.  I  must  next 
attempt  to  convey,  as  far  as  I  am  able,  the  correspond- 
ing conception  which  the  chemist  expresses  by  the  word 
atom.  The  terms  molecule  and  atom  are  constantly 
confounded ;  indeed,  have  been  frequently  used  as  sy- 
nonymous ;  but  the  new  chemistry  gives  to  these  words 
wholly  different  meanings.  We  have  already  defined  a 
molecule  as  the  smallest  mass  into  which  a  substance  is 
capable  of  being  subdivided  without  changing  its  chemi- 
cal nature  ;  but  this  definition,  though  precise,  does 
not  suggest  the  whole  conception ;  for  the  molecule 
may  be  regarded  from  two  very  different  points  of  view, 
according  as  we  consider  its  physical  or  its  chemical  re- 
lations. To  the  physicist,  the  molecules  are  the  points 
of  application  of  those  forces  which  determine  or  modify 
the  physical  condition  of  bodies,  and  he  defines  mole- 
cules as  the  small  particles  of  matter  which,  under  the 
influence  of  these  forces,  act  as  units.  Or,  limiting  his 
regards  to  those  phenomena  from  which  our  knowledge 
of  molecular  masses  is  chiefly  derived,  he  may  prefer  to 


CHEMICAL   DEFINITION   OF   MOLECULES.  99 

define  molecules  as  those  small  particles  of  bodies  which 
are  not  subdivided  when  the  state  of  aggregation  is 
changed  by  heat,  and  which  move  as  units  under  the 
influence  of  this  agent. 

To  the  chemist,  on  the  other  hand,  the  molecules 
determine  those  differences  which  distinguish  sub- 
stances. Sugar,  for  example,  has  the  qualities  which  we 
associate  with  that  name,  because  it  is  an  aggregate  of 
molecules  which  have  those  qualities.  Divide  up  a 
lump  of  sugar  as  much  as  you  please.  The  smallest 
mass  that  you  can  recognize  still  has  the  qualities  of 
sugar;  and  so  it  must  be,  if  you  continue  the  division 
down  to  the  molecule.  The  molecule  of  sugar  is  sim- 
ply a  very  small  piece  of  sugar.  Dissolve  the  sugar  in 
water,  and  we  obtain  a  far  greater  degree  of  subdivision 
than  is  possible  by  mechanical  means ;  a  subdivision 
which,  we  suppose,  extends  as  far  as  the  molecules. 
The  particles  are  distributed  through  a  great  mass  of 
liquid,  and  become  invisible ;  still,  the  qualities  of  the 
sugar  are  preserved  ;  and,  on  evaporating  the  water, 
we  recover  the  sugar  in  its  solid  condition  ;  and,  ac- 
cording to  the  chemist,  the  qualities  are  preserved,  be- 
cause the  molecules  of  sugar  have  remained  all  the 
while  unchanged. 

Consider,  in  the  second  place,  a  lump  of  salt.  You 
do  not  alter  its  familiar  qualities,  however  greatly  you 
may  subdivide  it,  and  the  molecules  of  salt  must  have 
all  the  saline  properties  which  we  associate  with  this 
substance.  Dissolve  the  salt  in  water,  and  you  simply 
divide  the  mass  into  molecules.  Convert  the  salt  into 
vapor,  as  you  readily  can,  and  again  you  isolate  the 
molecules  as  before.  But,  through  all  these  changes, 
the  salt  remains  salt ;  it  does  not  lose  its  savor,  because 
the  individuality  of  the  molecules  is  preserved.  So  is 


100  CHEMICAL   COMPOSITION. 

it  with  every  substance.  It  is  the  molecules  in  which 
the  qualities  inhere.  Hence  the  chemist's  definition  of 
a  molecule :  The  smallest  particles  of  a  substance  in 
which  its  qualities  inhere,  or  the  smallest  particles  of  a 
substance  which  can  exist  T)y  themselves  /  for  both  defi- 
nitions are  essentially  the  same. 

Hitherto  we  have  only  considered  molecules  as  dif- 
fering from  each  other  in  weight,  and  have  learned  how 
to  determine  their  weight ;  but  now  we  have  to  regard 
them  as  differing  in  all  those  qualities  which  distinguish 
substances.  Considering  only  the  ordinary  chemical 
relations  of  the  two  substances,  a  molecule  of  sugar  dif- 
fers from  a  molecule  of  salt  in  precisely  the  same  way 
that  a  lump  of  sugar  differs  from  a  lump  of  salt.  In  a 
word,  what  is  true  of  the  substance  in  mass  is  true  of 
its  molecules.  Hence  it  is  that,  in  studying  the  chemi- 
cal relations  of  substances,  we  may,  as  a  rule,  confine 
our  attention  to  the  relations  between  their  molecules, 
and  this  very  greatly  simplifies  the  problems  with 
which  we  have  to  deal ;  and,  in  the  admirable  system  of 
chemical  notation,  to  which  I  shall  hereafter  call  your 
attention,  the  symbol  of  a  substance  stands  for  one 
molecule,  and  in  using  these  symbols  to  represent  chemi- 
cal changes — reactions,  as  we  call  them — we  always  ex- 
press the  reaction  as  taking  place  between  the  individ- 
ual molecules  of  the  substances  concerned. 

But,  although  the  molecules  are  the  limit  of  the 
physical  subdivision  of  a  substance,  the  chemist  carries 
the  subdivision  still  further ;  but,  then,  the  parts  ob- 
tained have  no  longer  the  qualities  of  the  original  sub- 
stance, and  one  or  more  new  substances  result.  Of 
course,  the  chemist  cannot,  any  more  than  the  physi- 
cist, experiment  on  individual  molecules.  He  must 
experiment  on  a  mass  of  the  substance,  and  the  division 


HOW  MOLECULES  BECOME  DIVIDED.  101 

of  the  molecule  must  be  an  inference  from  the  phe- 
nomena which  ensue.  Let  me  call  your  attention  to  a 
few  experiments  which  will  illustrate  this  point : 

I  crush  this  lump  of  sugar  in  a  mortar,  and  reduce 
it  to  what  appears  to  be  an  impalpable  powder,  but  a 
microscope  wTill  show  that  the  powder  consists  of  grains 
which  are  simply  smaller  lumps,  and,  in  fact,  masses  of 
great  size,  compared  with  many  organisms  which  are 
the  objects  of  microscopic  investigation.  Each  one  of 
these  grains  is  sugar,  and  has  all  the  essential  qualifies 
of  sugar  just  as  much  as  the  lump.  We  next  pour  the 
powdered  sugar  into  water,  in  which,  as  we  say,  it  dis- 
solves ;  but  the  solution  simply  consists  in  dividing  the 
grains  still  more,  reducing  them  to  molecules,  which 
become  spread  throughout  the  mass  of  the  liquid.  How 
are  we  to  go  any  further  than  this  ?  Very  easily.  I  take 
a  few  more  lumps  of  sugar,  and  throw  them  into  this 
heated  platinum  crucible,  when,  in  an  instant,  a  re- 
markable change  takes  place.  We  have  the  appearance 
of  flame,  and  out  of  the  sugar  is  evolved  a  mass  of  loose 
charcoal.  Evidently,  this  charcoal  must  have  come 
from  the  sugar.  The  crucible  is  unchanged,  and,  be- 
sides the  air,  the  sugar  and  platinum  were  the  only 
substances  present.  Let  me,  however,  enforce  this  con- 
clusion by  still  another  experiment,  which  is  even  more 
striking : 

Instead  of  acting  on  the  sugar  simply  with  heat,  we 
will  now  act  upon  it  with  a  strong  chemical  agent  called 
sulphuric  acid.  For  this  purpose  I  have  previously  pre- 
pared about  half  a  pint  of  very  thick  syrup,  and  with  this 
I  will  now  mix  three  or  four  times  its  volume  of  common 
oil  of  vitriol,  constantly  stirring  the  mass  as  my  assist- 
ant pours  in  the  acid.  The  syrup  at  once  blackens ; 
soon  it  begins  to  swell,  and  now  notice  this  enormous 


102  CHEMICAL  COMPOSITION. 

body  of  loosely-coherent  charcoal  which  rises  from  the 
vessel.  Here,  again,  the  charcoal  must  have  been 
evolved  out  of  the  sugar,  for  the  sugar  was  the  only 
substance  common  to  the  two  experiments ;  and,  ad- 
mitting this  fact,  see  to  what  it  leads. 

The  qualities  of  sugar  inhere  in  its  smallest  particles, 
and  must  belong  to  the  molecules  just  as  truly  as  to 
these  lumps.  In  our  experiment  the  charcoal  has  been 
evolved  out  of  a  considerable  mass  of  sugar ;  but  the 
result  would  have  been  the  same  could  we  experiment 
on  the  individual  molecules.  It  is  evident,  therefore, 
that  the  charcoal  has  been  formed  out  of  the  sugar- 
molecules,  and  that  each  molecule  has  contributed  its 
portion  to  this  result.  Now,  this  charcoal,  although  so 
bulky,  weighs  far  less  than  the  sugar.  It  could,  then, 
have  formed  only  a  part  of  the  mass  of  the  sugar,  and 
only  a  part  of  the  mass  of  each  molecule.  But  what 
has  become  of  the  rest  of  the  material  ?  For  the  pres- 
ent, it  must  be  sufficient  to  state  that  careful  experi- 
menting has  shown  that,  in  this  process,  another  sub- 
stance is  evolved  from  the  sugar  besides  charcoal,  and 
that  this  substance  is  water.  Moreover,  since  the  weight 
of  the  water,  added  to  that  of  the  charcoal,  entirely  ac- 
counts for  the  material  of  the  sugar,  we  conclude  that 
in  our  experiment  the  sugar  has  been  resolved  solely 
into  charcoal  and  water.  Each  molecule,  therefore,  has 
been  resolved  into  charcoal  and  water.  In  a  word,  the 
molecule  has  been  divided.  We  cannot  divide  it  by 
any  physical  means  ;  but  we  can  divide  it  by  chemical 
means,  only  we  do  not  obtain  thereby  two  smaller  par- 
ticles of  sugar,  but  a  particle  of  charcoal  and  a  particle 
of  water.  Such,  then,  is  the  evidence  we  have  that 
a  molecule  of  sugar  can  be  divided ;  but  the  reason- 
ing here  used  is  so  important  to  the  validity  of  our 


DIVISION   OF   THE   MOLECULE   OF  WATER.  103 

modern  chemical  philosophy  that  I  must  not  pass  it  by 
with  a  single  example : 

One  of  the  substances  evolved  from  the  sugar  was 
water.  Let  us  next  see  whether  the  molecules  of  this 
most  familiar  substance  can  be  divided.  We  have  al- 
ready seen  to  what  a  wonderful  degree  of  tenuity  we 
can  carry  the  mechanical  subdivision  of  this  material. 
The  film  of  a  soap-bubble,  just  before  it  bursts,  is  less 
than  T,Tnr£,oi5"o  °f  an  ^nc^  *n  thickness.  A  square  inch 
of  this  film  would  weigh  only  one  T/D^¥  of  a  grain. 
Now,  the  unaided  eye  can  easily  distinguish  the  j^-  of 
an  inch  in  length,  or  yo-.Vinr  °f  a  square  inch  of  area  or 
a  quantity  of  water  in  that  film,  weighing  only  T¥,TRro7nr 
of  a  grain.  But  a  still  greater  subdivision  than  this  is 
possible,  for,  as  we  now  know,  when  water  is  converted 
into  vapor,  the  liquid  mass  breaks  up  into  small  parti- 
cles of  wonderful  tenuity,  which  we  call  molecules,  and 
by  expanding  the  vapor  we  can  separate  these  molecules 
to  an  indefinite  extent.  We  cannot,  it  is  true,  follow 
this  subdivision  with  the  eye,  but  we  can  discern  it 
with  the  intellect ;  and,  furthermore,  by  determining 
the  specific  gravity  of  aqueous  vapor  with  reference  to 
hydrogen  gas,  we  can  very  easily  find  the  weight  of 
the  aqueous  molecules,  and  we  thus  know  that  a  mole- 
cule of  water  weighs  eighteen  microcriths.  By  physical 
processes  we  cannot  carry  the  subdivision  any  further. 
The  smallest  mass  of  water  of  which  we  have  any  knowl- 
edge weighs  eighteen  microcriths ;  but  we  can  divide 
the  molecule  chemically,  as  the  following  experiment 
will  prove : 

In  order  to  show  you  the  decomposition  of  water 
by  an  electrical  current,  I  have  projected  on  the  screen 
the  magnified  image  of  a  glass  cell  containing  a  small 
quantity  of  this  familiar  liquid,  acidulated,  however 


104  CHEMICAL   COMPOSITION. 

(with  sulphuric  acid),  in  order  to  make  it  a  conductor 
of  electricity.  Connected  with  the  cell  is  what  must 
be  known  to  all  of  my  audience  as  a  voltaic  battery. 
The  conducting  wires  from  the  end  plates  of  this  com- 
bination terminate  in  the  two  strips  of  platinum,  which 
you  see  projected  on  the  screen.  As  soon  as  the  con- 
nections are  made,  or,  to  use  the  technical  phrase,  as 
soon  as  the  circuit  is  closed,  an  electric  current  flows 
through  the  water  in  the  cell,  passing  from  one 
of  these  poles  to  the  other.  The  effect  of  this  current 
is  visible.  Bubbles  of  gas  collect  upon  the  platinum 
strips,  and,  as  soon  as  they  attain  sufficient  size,  rise  to 
the  surface  of  the  water,  and  this  evolution  of  gas  will 
go  on  so  long  as  the  electric  current  continues  to  flow. 
The  gases  evolved  at  the  two  poles  are  wholly  different 
substances,  and,  in  order  to  exhibit  to  you  their  charac- 
teristic qualities,  I  have  prepared  a  second  experiment : 

Standing  on  the  table  is  a  decomposing  cell  similar 
to  the  last,  but  very  much  larger,  and  so  constructed 
that  the  two  gases  are  collected  as  they  rise  from  the 
poles,  and  conducted  apart  into  these  two  glass  bells. 
A  very  powerful  electric  current  has  been  passing 
through  the  water  in  the  cell  since  the  beginning  of 
the  lecture,  and  already  the  bells  are  filled  with  the 
two  aeriform  products.  Both  are  invisible,  but  notice 
that  the  gas  we  have  collected  in  the  right-hand  bell 
takes  fire  and  burns  with  a  pale  and  barely  luminous 
flame.  Here  we  have  a  very  large  bell  full  of  the  same 
gas,  and  on  lighting  this  I  think  the  flame  will  be  visi- 
ble to  all.  Every  one  must  have  recognized  this  ma- 
terial. 

It  is  a  substance  which  we  call  hydrogen,  a  gas  that 
retains  its  aeriform  condition  more  persistently  than  any 
other  material  with  which  we  are  acquainted.  It  is, 


COMPOSITION   OF   WATER.  105 

moreover,  the  lightest  form  of  matter  known.  A  cubic 
yard  of  air  at  the  temperature  of  this  room  (77°  Fahr.) 
weighs,  in  round  numbers,  two  pounds,  while  a  cubic 
yard  of  hydrogen  weighs  only  two  and  a  half  ounces. 
These  rubber  balloons,  which  are  such  familiar  toys, 
illustrate  very  forcibly  the  wonderful  lightness  of  this 
singular  form  of  matter. 

Let  us  turn  now  to  the  gas  in  the  left-hand  bell,  and 
we  shall  find  that  it  differs  most  strikingly  from  the 
other,  and  in  no  respect  is  the  difference  more  marked 
than  in  the  weight.  This  gas  is  sixteen  times  heavier 
than  hydrogen,  that  is,  the  difference  between  the  den- 
sity of  the  two  is  almost  as  great  as  that  between  iron 
and  cork,  and  yet  these  invisible  forms  of  matter  are  so  in- 
tangible that  it  is  difficult  even  for  the  chemist  to  appreci- 
ate this  difference.  Bringing  now  a  lighted  candle  netar 
the  open  mouth  of  the  bell,  you  see  that  the  gas  will  not 
burn ;  but  notice  that,  as  I  lower  the  candle  into  the 
bell,  the  wax  burns  in  the  gas  far  more  brilliantly  than 
in  air.  Observe,  also,  that  this  smouldering  slow-match 
bursts  into  flame  when  immersed  in  the  same  medium. 
Evidently  it  supports  combustion  with  great  vigo  , 
and,  in  order  to  illustrate  this  point  still  more  strik- 
ingly, I  will  introduce  into  another  bell  of  the  same  gas 
a  spiral  of  watch-spring  tipped,  like  a  match,  with  a  lit- 
tle sulphur,  first  setting  fire  to  the  sulphur.  .  .  .  See ! 
the  iron  burns  as  readily  as  tinder,  and  far  more  brill- 
iantly. We  are  dealing,  in  fact,  with  oxygen,  the  same 
gas  which  is  found  all  around  us  in  the  earth's  atmos- 
phere— only,  in  our  atmosphere  the  oxygen  is  mixed 
with  four  times  its  volume  of  an  inert  gas  called  nitro- 
gen, while  as  evolved  from  the  water  in  our  experiment 
it  is  perfectly  pure. 

It   is   evident,  then,  that   in  this  experiment  two 


106  CHEMICAL   COMPOSITION. 

new  substances  are  evolved,  and  the  question  arises, 
Whence  do  they  come  ?  If  we  examine  carefully 
the  conditions  of  the  experiment  we  should  find 
that,  of  all  the  substances  present,  the  only  one  which 
underwent  any  permanent  change  was  the  water.  The 
weight  of  the  platinum  poles,  for  example,  remains  un- 
changed, but  the  weight  of  the  water  is  diminished  in 
exact  proportion  to  the  amount  of  gas  evolved.  These 
aeriform  substances  are  then  educed  from  the  material 
of  the  water.  Moreover,  it  has  also  been  proved  that 
the  water  is  completely  resolved  into  these  gases.  The 
electric  current  is  merely  a  form  of  energy,  and,  of 
course,  can  neither  add  nor  remove  ponderable  mate- 
rial, and  the  weight  of  oxygen  and  hydrogen  formed  is 
exactly  equal  to  the  weight  of  water  lost.  As  we  say  in 
chemistry,  the  electric  current  analyzes  the  water,  and 
these  gases  are  its  sole  constituents. 

Let  me  now  call  your  attention  to  another  fact  con- 
nected with  the  process  we  are  studying;  and,  in  order 
that  you  rfiay  observe  the  fact  for  yourselves,  I  will  re- 
peat the  experiment  with  still  a  third  apparatus,  so 
constructed  that  we  can  measure  the  volumes  of  the 
two  gases  which  are  formed.  I  have  placed  the  appa- 
ratus in  front  of  my  lantern  so  that  I  can  project  on 
the  screen  a  magnified  image  of  the  graduated  tubes  in 
which  the  gases  are  collected. 

You  notice  that  the  volume  of  one  is  twice  as  large 
as  that  of  the  other,  and  this  ratio  is  found  to  hold  ex- 
actly when  we  make  the  experiment  with  the  very 
greatest  accuracy.  The  larger  volume  is  hydrogen,  the 
lesser  oxygen.  But  oxygen,  as  I  have  said,  is  sixteen 
times  as  heavy  as  hydrogen.  Hence,  there  is  eight 
times  as  much  material  in  the  half-volume  of  oxygen  as 
in  the  whole  volume  of  hydrogen,  or,  in  other  words, 


COMPOSITION   OF   WATER.  107 

when  water  is   decomposed  by   electrolysis,  there  is 
eight  times  as  much  oxygen  produced  as  hydrogen. 

We  regard,  then,  this  experiment  as  establishing, 
beyond  all  controversy,  the  fact  that  water  is  composed 
of  oxygen  and  hydrogen  gases  in  the  proportions  of 


FIG.  21. — Decomposition  of  Water  by  Galvanism. 

eight  to  one,  or,  in  other  words,  that  in  every  nine 
parts  of  water  there  are  eight  parts  of  oxygen* and  one 
part  of  hydrogen.  But,  if  this  is  true,  it  must  be  true 
of  the  smallest  mass  of  water  as  well  as  of  the  largest. 
It  must  be  true,  then,  of  the  molecule  of  water.  Now, 
one  molecule  of  water  weighs  18  microcriths.  Hence, 
of  those  18  microcriths,  one-ninth,  or  two  microcriths, 
must  consist  of  hydrogen,  and  eight-ninths,  or  16  micro- 
criths, must  consist  of  oxygen.  Please  notice  that  this 
is  a  result  to  which  our  experiment  directly  leads,  and 
is  as  much  a  fixed  truth  as  any  results  of  observation. 
Unless  our  whole  science  is  in  error,  and  Avogadro's 
law  a  delusion,  then  it  is  an  established  fact  that  the 
molecule  of  water  weighs  18  microcriths,  and  equally 
certain  that  this  molecule  consists  of  16  microcriths  of 
oxygen,  and  of  2  microcriths  of  hydrogen.  More- 


108  CHEMICAL  COMPOSITION. 

over,  it  is  also  evident  that,  when  we  analyze  water,  as 
in  this  experiment,  the  molecules  are  divided,  and  that, 
from  the  material  thus  obtained  are  formed  the  mole- 
cules of  the  two  aeriform  substances  which  are  the 
products  of  the  process.  As  yet  I  advance  no  theory 
as  regards  the  nature  of  this  process,  or  of  the  condi- 
tion in  which  the  two  substances  exist  in  the  molecule 
of  water.  I  am  only  dealing  with  the  bare  fact  that 
they  are  evolved  out  of  the  molecule,  and  that  the 
molecule  is  thus  divided.  There  are  a  great  many 
other  chemical  processes  by  which  water  may  be  ana- 
lyzed, and  the  result  is  in  all  cases  precisely  the  same, 
namely,  that  from  every  nine  parts  of  water  there  are  ob- 
tained eight  parts  of  oxygen  and  one  of  hydrogen.  Of 
course  this  concurrence  of  testimony  is  very  valuable, 
but  we  need  not  go  beyond  this  simple  experiment  to 
establish  the  truth  we  have  enunciated,  and  our  experi- 
ment has  this  great  advantage  for  the  present  purpose  : 
There  is  nothing  to  complicate  the  process,  and  you 
can  be  almost  said  to  see  that  the  oxygen  and  hydro- 
gen come  from  the  water  and  from  that  alone.  Such 
illustrations  might  be  very  greatly  multiplied,  but  the 
two  we  have  selected  are  sufficient  to  show  how  the 
chemist  is  able  to  divide  the  molecule,  and  that  this 
division  is  always  attended  with  the  destruction  of  the 
original  substance,  and  the  evolution  from  it  of  wholly 
different  substances. 

As  we  saw  at  the  last  lecture,  the  very  essence  of  a 
chemical  process  consists  in  the  conversion  of  the  sub- 
stances we  called  the  factors  into  new  substances  we 
called  the  products ;  and  it  now  appears  that  all  such 
changes  imply  a  destruction  of  the  original  molecules, 
and  the  formation  of  new  molecules  from  the  same 
materials.  The  original  molecules  are  destroyed  ;  there- 


CHEMICAL   CHANGES,   HOW   DEFINED.  109 

fore  the  original  substances  disappear.  New  molecules 
are  formed ;  hence  new  substances  result. 

Even  at  the  cost  of  repetition  let  us  make  sure  that 
we  fully  comprehend  this  reasoning  on  which  the  whole 
molecular  theory  of  chemistry  rests.  What  we  observe 
is,  that  in  one  chemical  process  there  come  from  the 
material  of  sugar,  for  example,  charcoal  and  water,  and 
that  in  another  chemical  process  there  come  from  the 
material  of  water  oxygen  and  hydrogen  gases ;  and  we 
reason  that  each  molecule  of  the  sugar,  or  each  molecule 
of  the  water,  must  have  contributed  its  share  toward  the 
formation  of  the  several  products ;  and,  further,  that  each 
molecule  of  the  products  must  have  been  made  from  the 
dissevered  parts  of  the  molecules  of  the  original  factors. 
Of  course,  this  reasoning  assumes  the  fundamental  con- 
cept of  the  molecular  theory,  namely,  that  of  every  sub- 
stance there  are  definite  ultimate  particles  in  which  the 
qualities  inhere,  and  which  cannot  be  divided  without 
destroying  the  substance.  But,  assuming  the  reality  of 
the  concept,  the  conclusion  that  in  every  chemical  process 
molecules  are  either  divided  or  constructed,  and  usually 
both  divided  and  constructed,  is  a  necessary  inference. 

In  some  cases  the  old  molecules  are  divided  into 
parts  of  a  different  nature.  Thus,  the  molecules  of 
sugar  are  divided  into  masses  of  charcoal  and  water,  and 
the  molecules  of  water  again  are  divided  into  particles 
of  oxygen  and  hydrogen.  In  such  cases,  we  say  that 
the  substance  is  decomposed  into  its  constituent  parts. 
In  other  cases,  the  old  molecules  attach  to  themselves 
more  material,  and  new  molecules,  of  greater  weight, 
result,  and  we  then  say  that  the  substance  has  com- 
bined with  another,  as  the  coal  with  oxygen  in  the  pro- 
cess of  burning,  and  the  iron  with  oxygen  in  the  pro- 
cess of  rusting.  The  first  class  of  changes  we  call 


110  ANALYSIS  AND   SYNTHESIS. 

analysis,  the  second,  synthesis.  The  evidence  of  analy- 
sis is  that  each  product  of  the  change  weighs  less  than 
the  substance  from  which  it  was  evolved.  The  evi- 
dence of  synthesis  is  that  the  total  product  weighs 
more  than  the  original  substance. 

The  oxygen  and  hydrogen  gases,  each  apart,  weigh 
less  than  the  water  from  which  they  were  formed,  and 
the  fact  that  the  sum  of  their  weights  is  exactly  equal 
to  that  of  the  wrater,  proves  that  they  are  the  only 
products  of  the  change,  and  that  water  is  composed  of 
these  substances,  and  of  these  alone.  The  gas  we  call 
carbonic  dioxide,  which  is  the  only  product  of  the 
burning  of  pure  coal,  weighs  more  than  the  coal,  and, 
since  this  excess  of  weight  is  exactly  equal  to  that  of 
the  oxygen  consumed  in  the  burning,  we  conclude  that, 
in  this  process,  the  coal  has  combined  with  oxygen, 
and  that  the  carbonic  dioxide  is  a  compound  of  these 
two  substances. 

Thus  arise  our  scientific  conceptions  of  combina- 
tion and  decomposition,  of  synthesis  and  analysis. 
When  we  say  that  sugar  is  composed  of  charcoal  and 
water,  we  mean  merely  that  these  two  substances  may 
be  evolved  from  sugar ;  and  the  evidence  that  they  are 
the  only  constituents  of  sugar  is  that  the  sum  of  the 
weights  of  the  two  products  equals  the  weight  of  the 
sugar.  When  we  say  that  water  is  composed  of  oxy- 
gen and  hydrogen,  we  merely  mean  that  these  two 
substances  may  be  educed  from  water,  and  that,  as  be- 
fore, the  weight  of  the  two  products  exactly  equals  the 
weight  of  the  water.  When  we  say  that  carbonic  di- 
oxide is  composed  of  charcoal  and  oxygen,  our  asser- 
tion is  based  on  the  fact  that,  in  the  process  of  burning, 
the  oxygen  gas  appears  to  absorb  charcoal,  and  that 
the  resulting  gas  weighs  more  than  the  oxygen  by  the 


WEIGHT   THE   MEASURE   OF   MATERIAL.  m 

exact  weight  of  the  charcoal  consumed.  In  the  first 
two  cases,  the  proof  of  the  composition  is  analytical, 
in  the  third  synthetical.  In  many  cases  we  have  both 
modes  of  proof.  Thus,  we  can  decompose  water  into 
oxygen  and  hydrogen  gases,  and  show  that  the  weight 
of  the  products  is  exactly  the  same  as  that  of  the  water 
which  has  disappeared.  We  can  also  combine  hydro- 
gen with  oxygen,  and  show  that  the  weight  of  water 
formed  is  exactly  equal  to  that  of  the  two  gases  con- 
sumed. 

Notice  the  important  part  which  the  weight  of  the 
substances  concerned  in  our  processes  plays  in  this 
reasoning.  That  water  consists  of  oxygen  and  hydrogen, 
and  of  nothing  else,  is  a  conclusion  based  on  the  fact 
that  the  weight  of  the  substance  has  been  found  equal 
to  that  of  its  assumed  constituents.  Of  course  the 
reasoning  implies  the  truth  of  the  great  principle  of 
the  conservation  of  mass,  which  was  illustrated  at  the 
last  lecture.  It  is  simply  because  this  law  of  Nature  is 
fixed  and  unvarying  that  we  have  a  right  to  assume 
that  increase  of  weight  always  indicates  increase  of 
material,  and  diminution  of  weight  diminution  of  ma- 
terial ;  or,  in  other  words,  that  the  weight  of  a  body  is 
proportional  to  the  amount  of  material  it  contains.  It 
is  this  law  which  gives  us  confidence  throughout  all  the 
changes  of  substance  which  chemical  processes  involve, 
that  wherever  weight  has  been  gained,  material  has 
been  gained,  and  that  wherever  weight  has  been  lost, 
material  has  been  lost.  When  in  any  chemical  problem 
we  have  accounted  for  all  the  weight,  it  is  this  law 
which  gives  us  the  assurance  that  our  account  is  cor- 
rect ;  and,  on  the  other  hand,  when  the  account  does 
not  balance,  it  is  this  law  also  which  compels  us  to 
search  for  the  material  that  has  been  lost  or  gained. 


112  MERIT  OF  LAVOISIER. 

The  qualities  of  substances  are  evanescent,  but  under- 
lying these  qualities  is  something  which  alone  deter- 
mines weight.  We  sometimes  call  this  "substantia" 
mass,  we  sometimes  call  it  matter,  or  we  speak  of  it  as 
so  much  material ;  but  what  it  is  in  its  essence  we 
know  not.  This  much,  however,  we  do  know :  this 
essence  is  an  indestructible  quantity,  and  this  quantity 
is  measured  by  weight. 

But  this  great  law  of  the  conservation  of  mass,  so 
obvious  now,  is  by  no  means,  as  might  at  first  appear, 
self-evident,  and  it  is  only  comparatively  recently  that 
it  has  become  an  accepted  principle  of  science.  It  was 
really  implied  in  the  enunciation  of  the  great  law  of 
gravitation;  for  if  the  quantity  of  matter  in  a  body 
could  change,  in  consequence  of  any  chemical  action 
between  the  materials  of  which  it  consists,  then  the 
force  of  attraction  between  two  masses  of  matter  would 
depend  not  simply  on  their  distance  and  on  the  quan- 
tity of  matter  they  contained,  but  also  on  the  chemical 
condition  of  that  matter.  Moreover,  the  weight  of  an 
hermetically  sealed  vessel  might  be  altered  by  chemical 
action  within  its  walls.  But,  although  Newton  clearly 
conceived  that  weight  was  in  all  cases  proportional  to 
the  amount  of  material — whatever  its  form  or  condi- 
tion— and,  although  his  master-mind  was  able  to  estab- 
lish the  foundations  of  astronomy  on  this  basis  two 
centuries  ago,  it  is  only  comparatively  recently  that  the 
principle  has  been  fully  accepted  in  chemistry.  For 
years  after  Newton,  the  chemists  believed  universally 
in  a  kind  of  matter  called  phlogiston,  which  not  only 
could  be  removed  from  a  substance  without  diminish- 
ing its  weight,  but  whose  subtraction  actually  added  to 
the  weight.  It  is  the  great  merit  of  Lavoisier  that 


IMPONDERABLE   AGENTS.  113 

he  clearly  conceived  of  this  principle,  and  insisted  on 
its  application  in  chemistry.  He  was  the  first  to  see 
•  clearly  that,  in  every  chemical  process,  increase  of 
weight  means  increase  of  material,  and  loss  of  weight 
loss  of  material.  Iron,  in  rusting,  gains  in  weight. 
Hence,  said  Lavoisier,  it  has  combined  with  some 
material.  JSro,  said  the  defenders  of  the  phlogiston  the- 
ory, such  men  as  Cavendish,  Priestley,  and  Scheele, 
it  has  only  lost  phlogiston.  You  are  making  too  much 
of  this  matter  of  weight.  Phlogiston  differs  from  your 
gross  forms  of  matter  in  that  it  is  specifically  light, 
and,  when  taken  from  a  body,  increases  its  weight. 
We  smile  at  this  idea,  and  we  find  it  difficult  to  believe 
that  these  men,  the  first  scientific  minds  of  their  age, 
could  believe  in  such  absurdity.  But  we  must  remem- 
ber that  the  idea  did  not  originate  with  them.  It  was 
a  part  of  the  old  Greek  philosophy,  and  from  the  pages 
of  Aristotle  was  taught  in  every  school  of  Europe  until 
within  two  hundred  years ;  and,  even  in  our  own  time, 
we  still  hear  of  imponderable  agents.  Text-books  of 
science  are  used  in  some  of  our  schools  which  refer  the 
phenomena  of  heat  and  electricity  to  attenuated  forms 
of  matter,  that  can  be  added  to  or  subtracted  from 
bodies  without  altering  their  weight.  Such  facts  should 
teach  us,  not  that  we  are  so  much  wiser  than  our 
fathers,  but  that  our  familiar  ideas  of  the  composition 
of  matter  are  not  such  simple  deductions  from  the 
phenomena  of  Nature  as  they  appear  to  us;  and  this 
discussion  of  the  evidence,  on  which  these  conclusions 
are  based,  is  therefore  by  no  means  superfluous. 

As  the  result  of  this  discussion  let  us  bear  in  mind 
that,  when  we  say  that  water  is  composed  of  oxygen 
and  hydrogen,  we  mean  no  more  than  this,  that,  by 
various  chemical  processes,  these  two  substances  can 


114        COMBINATION   OF   OXYGEN  AND   HYDROGEN. 

be  produced  from  water,  and  that  the  weight  of  the 
two  products  always  equals  the  weight  of  the  water 
employed  in  the  process ;  or,  on  the  other  hand,  that 
water  may  be  produced  by  the  combination  of  oxygen 
with  hydrogen,  and  that  the  weight  of  the  water  thus 
formed  is  equal  to  the  sum  of  the  weights  of  the  two 
gases.  We  cannot  say  that  water  consists  of  hydrogen 
and  oxygen,  in  the  same  sense  that  bread  consists  of 
flour,  or  syrup  of  sugar,  and  mortar  of  lime.  We  must 
be  very  careful  not  to  transfer  our  ideas  of  composi- 
tion, drawn  chiefly  from  the  mixtures  we  use  in  com- 
mon life,  directly  to  chemistry.  In  these  mixtures  the 
product  partakes,  to  a  greater  or  less  degree,  of  the 
character  of  its  constituents,  which  can  be  recognized 
essentially  unchanged  in  the  new  material,  but,  in  all 
instances  of  true  chemical  union  and  decomposition, 
the  qualities  of  the  substances  concerned  in  the  process 
entirely  disappear,  and  wholly  different  substances, 
with  new  qualities,  appear  in  their  place.  Prior  to  ex- 
perience, no  one  could  suspect  that  two  aeriform  sub- 
stances like  oxygen  and  hydrogen  could  be  obtained 
from  water,  and  the  discovery  of  the  fact,  near  the  be- 
ginning of  this  century,  marks  an  era  in  the  history  of 
science.  And,  even  now,  familiar  as  it  is,  this  truth 
stands  out  as  one  of  the  most  remarkable  facts  of  Na- 
ture. Moreover,  the  wonder  becomes  still  greater 
when  we  learn  that  water  yields  1,800  times  its  vol- 
ume of  the  two  gases,  and  that  these  gases  retain 
their  aSriform  condition  so  persistently  that  mechanical 
pressure  alone  can  not  reduce  them  to  the  liquid  condi- 
tion ;  and  still  more  the  wonder  grows,  when  we  learn 
further  that  the  amount  of  energy  required  to  decom- 
pose a  pound  of  water  into  its  constituent  gases  would 
be  adequate  to  raise  a  weight  of  5,314,200  pounds  one 


ENERGY  DEVELOPED.  115 

foot  high ;  and  that,  when  these  gases  unite  and  the 
water  is  reproduced,  this  energy  again  becomes  active. 
Two  experiments  will  enforce  the  truth  of  this  state- 
ment: 

For  the  first,  I  have  mixed  together  in  this  rubber 
bag  oxygen  and  hydrogen  in  the  exact  proportions  in 
which  they  unite  to  form  water,  and,  with  the  gas,  I 
will  now  blow  up  into  froth  the  soap-suds  contained  in 
this  iron  mortar — thus  confining  the  gas  only  by  the 
thinnest  possible  envelope.  I  will  now  ask  my  assist- 
ant to  inflame  the  mixture  with  his  lighted  taper,  when 
a  deafening  explosion  announces  to  us  that  the  chemi- 
cal union  has  taken  place.  But  what  has  been  the 
occasion  of  the  development  of  such  tremendous  ener- 
gy ?  The  formation  of  a  single  drop  of  water,  so  small 
that  you  could  hold  it  on  the  point  of  a  needle. 

For  the  second  experiment  I  will  burn  the  same 
gas-mixture  at  a  jet,  and  show  you  how  great  is  the  in- 
tensity of  the  heat  which  may  be  thus  developed.  This 
apparatus  is  the  well-known  compound  blow-pipe  in- 
vented by  our  countryman  Dr.  Hare.  The  oxygen 
and  hydrogen  flow  through  rubber  hose  from  separate 
gas-holders  into  a  very  small  chamber,  where  they  mix 
before  issuing  from  the  jet.  The  same  chemical  union 
takes  place  here  as  before ;  the  same  product  (water)  is 
formed ;  the  same  amount  of  energy  is  developed ;  but, 
under  these  different  conditions,  the  explosive  gas 
burns  with  a  quiet  flame  as  it  is  gradually  supplied 
from  the  jet,  and  the  energy,  instead  of  being  expended 
in  driving  back  the  air,  and  thus  determining  that  vio- 
lent commotion  in  the  atmosphere  which  caused  the 
noise,  is  here  manifested  wholly  as  heat.  And  see  how 
intense  the  heat  is !  ...  It  is  a  steel  file  which  is  burn- 
ing with  such  rapidity  in  this  flame.  As  I  have  already 


116  WHAT   WE  KNOW. 

told  you,  heat  is  only  a  mode  of  energy,  and,  like  any 
other  manifestation  of  power,  may  be  measured  in  foot- 
pounds. Hence,  this  brilliant  experiment  is  an  apt 
illustration  of  the  amount  of  energy  developed  in  the 
production  of  water.  In  witnessing  the  magnitude  of 
the  effects,  we  are  surprised,  as  before,  by  the  apparent 
inadequacy  of  the  cause;  for  the  amount  of  water, 
whose  production  was  the  occasion  of  all  this  display 
of  power,  is  only  a  few  drops. 

Who  could  believe  that  such  power  was  concealed 
in  the  familiar  liquid  which  is  so  intimately  connected 
with  our  daily  life  ?  Between  the  qualities  of  water  and 
the  qualities  of  these  gases  there  is  not  the  most  distant 
resemblance.  When  the  water  is  decomposed,  the 
qualities  of  the  water  are  wholly  lost  in  the  qualities 
of  the  two  gases  produced  from  it,  and  a  certain  amount 
of  energy  is  absorbed.  When  the  water  is  formed,  the 
qualities  of  oxygen  and  hydrogen  are  wholly  merged 
in  those  of  the  resulting  liquid,  while  the  same  amount 
of  energy  is  set  free.  Whether  the  oxygen  and  hydro- 
gen exist,  as  such,  in  the  water,  or  whether  they  are 
produced  by  some  unknown  and  unconceived  transfor- 
mation of  its  substance,  is  a  question  about  which  we 
may  speculate,  but  in  regard  to  which  we  have  no 
knowledge.  All  we  know  is,  that  the  change  of  water 
into  the  two  gases  or  of  the  two  gases  into  water  is 
attended  with  no  change  of  weight,  and  hence  we  con- 
clude that  in  the  change  the  material  is  preserved,  or, 
in  other  words,  that  water  and  the  gases  are  the  same 
material  in  different  forms. 

Now,  the  only  theory  which  has  as  yet  succeeded  in 
giving  an'intelligible  explanation  of  the  facts,  assumes 
that  hydrogen  and  oxygen  do  exist  as  such  in  water, 
preserving  each  its  individuality ;  that  each  molecule 


THE   ATOMIC   THEORY.  117 

of  water  consists  of  three  particles,  two  of  hydrogen 
and  one  of  oxygen ;  that,  when  the  water  is  decom- 
posed, the  molecules  are  broken  up,  and  that  then  the 
oxygen  particles  associate  themselves  together  to  form 
molecules  of  oxygen  gas,  and  the  hydrogen  particles  to 
form  molecules  of  hydrogen  gas ;  that,  on  the  other 
hand,  when  the  gases  recombine,  the  reverse  takes 
place,  each  particle  of  oxygen  uniting  to  itself  two  par- 
ticles of  hydrogen  to  form  a  molecule  of  water. 

These  parts  of  molecules  (these  particles,  into 
which  the  molecules  break  up  under  various  chemical 
processes)  are  what  we  call  atoms,  and  this  theory  is 
the  famous  atomic  theory,  which  has  played  such  a 
prominent  part  in  modern  chemistry.  We  shall  find, 
as  we  proceed,  that  there  is  very  strong  evidence  in  its 
support.  Indeed,  without  it  a  large  part  of  the  mod- 
ern science  would  be  wholly  unintelligible ;  and,  were 
I  to  confine  my  regards  to  purely  chemical  facts,  I 
should  regard  the  evidence  in  its  favor  as  overwhelm- 
ing. Still,  I  must  confess  that  I  am  rather  drawn  to 
that  view  of  Nature  which  has  favor  with  many  of  the 
most  eminent  physicists  of  the  present  time,  and  which 
sees  in  the  cosmos,  besides  mind,  only  two  essentially 
distinct  beings,  namely,  matter  and  energy,  which  re- 
gards all  matter  as  one  and  all  energy  as  one,  and 
which  refers  the  qualities  of  substances  to  the  affections 
of  the  one  substratum,  modified  by  the  varying  play 
of  forces.  According  to  this  view,  the  molecules  of 
water  are  perfectly  homogeneous,  and  the  change, 
which  takes  place  when  water  is  decomposed,  does  not 
consist  in  the  separation  from  its  molecules  of  pre- 
existing particles,  but  in  imparting  to  the  same  mate- 
rial other  affections. 

I  know  that  this  language  is  very  vague,  but  it  is- 


118  THE  ATOMIC  THEORY. 

no  more  vague  than  the  idea  it  attempts  to  embody. 
Still,  vague  as  it  is,  no  one  who  has  followed  modern 
physical  discussions  can  doubt  that  the  tendency  of 
physical  thought  is  to  refer  the  differences  of  substances 
to  a  dynamical  cause.  Nevertheless,  as  1  said  before, 
the  atomic  theory  is  the  only  one  which,  as  yet,  has 
given  an  intelligible  explanation  of  the  facts  of  modern 
chemistry,  and  I  shall  next  proceed  to  develop  its  fun- 
damental principles.  I  wish,  however,  before  I  begin, 
to  declare  my  belief  that  the  atomic  theory,  beautiful 
and  consistent  as  it  appears,  is  only  a  temporary  expedi- 
ent for  representing  ttie  facts  of  chemistry  to  the  mind. 
Although  in  the  present  state  of  the  science  it  gives 
absolutely  essential  aid  both  to  investigation  and  study, 
I  have  the  conviction  that  it  is  a  temporary  scaffolding 
around  the  imperfect  building,  which  will  be  removed 
as  soon  as  its  usefulness  is  passed.  I  have  been  called  a 
blind  partisan  of  the  atomic  theory,  but,  after  this  dis- 
claimer, you  will  understand  me  when,  during  the  re- 
mainder of  this  course  of  lectures,  I  shall  endeavor  to 
present  its  principles  as  forcibly  as  I  can. 


LECTUEE  VI. 

ELEMENTAEY   SUBSTANCES    AND   COMBINING    PKOPORTIONS. 

IN  my  last  lecture  I  stated  that  in  a  chemical  com- 
pound the  qualities  of  the  constituents  are  wholly  merged 
in  those  of  the  product,  and  that  this  circumstance  dis- 
tinguishes a  true  compound  from  a  mechanical  mixture 
in  which  the  qualities  of  each  ingredient  are  to  a  greater 
or  less  extent  preserved.  This  distinction  is  one  of 
very  great  importance  in  chemistry,  and  I  will  begin 
my  lecture  this  evening  by  asking  your  attention  to  a 
simple  experiment,  which  will  recall  the  principal 
points  of  our  discussion  at  the  last  lecture  and  at  the 
same  time  illustrate  still  other  aspects  of  this  impor- 
tant subject. 

I  have  prepared  a  mixture  of  finely-divided  iron 
(iron  reduced  by  hydrogen)  and  flowers  of  sulphur. 
The  two  powders  have  been  rubbed  together  in  a  mor- 
tar until  the  mass  appears  perfectly  homogeneous  and 
it  is  impossible  with  the  unaided  eye  to  distinguish  the 
grains  of  either  substance,  and  yet  nothing  is  easier 
than  to  show  that  both  are  here  wholly  unchanged. 

For  this  purpose  I  will,  in  the  first  place,  pour  upon 
a  portion  of  the  powder  some  of  this  colorless  liquid 
called  sulphide  of  carbon,  which  dissolves  sulphur  with 
great  eagerness.  After  shaking  the  two  together  we 


120  ELEMENTARY  SUBSTANCES. 

find  left  on  the  bottom  of  our  glass  beaker  a  quantity 
of  a  black  powder,  which,  as  the  magnet  shows  at 
once,  is  iron.  In  the  second  place  I  will  stir  up 
another  portion  of  the  mixture  with  alcohol,  using  this 
liquid  to  hold  the  powder  in  suspension  so  that  I  can 
pick  out  the  grains  of  iron  with  a  magnet.  Using  this 
bar-magnet  as  a  stirring-rod,  I  can  thus  readily  wash 
out  the  sulphur  from  the  iron  which  adheres  to  the 
magnet,  and  we  recognize  at  once  the  yellow  color  as 
the  particles  of  sulphur  settle  to  the  bottom  of  the  jar. 

Having  shown  you  now  that  both  iron  and  sulphur 
are  here  present,  with  their  qualities  wholly  unaltered, 
I  will  next  take  a  third  portion  of  the  powder,  and, 
having  made  with  it  a  small  conical  heap,  apply  a 
lighted  match  to  the  apex  of  the  cone.  A  glow  at 
once  spreads  through  the  whole  mass,  which  is  an  evi- 
dence to  me  that  a  chemical  change  has  taken  place, 
and  in  that  change  the  sulphur  and  iron  have  disap- 
peared. The  mass  has  somewhat  caked  together,  but 
we  can  easily  pulverize  it  again,  and  our  product  is 
then  a  black  powder  not  differing  very  greatly  in  ex- 
ternal appearance  from  the  original  material.  But  from 
this  black  powder  the  sulphide  of  carbon  can  dissolve 
no  sulphur,  and  the  magnet  can  remove  no  iron. 

The  qualities  both  of  the  iron  and  the  sulphur  have 
disappeared,  and  those  of  a  new  substance  we  call  sul- 
phide of  iron  have  taken  their  place,  and  the  only  evi- 
dence we  have  that  the  material  of  the  sulphur  and  the 
material  of  the  iron  are  still  here  is  the  weight  of  the 
sulphide  of  iron,  which  is  exactly  equal  to  that  of  the 
sulphur  and  iron  combined.  So  long  as  the  sulphide 
of  iron  remains  sulphide  of  iron,  no  scrutiny  can  de- 
tect in  it  either  sulphur  or  iron,  and  we  must  have  re- 
course to  other  chemical  processes  in  order  to  repro- 


CHEMICAL   COMBINATION.  121 

duce  these  substances.  In  old  times,  before  men  had 
clearly  conceived  that  weight  is  the  measure  of  mate- 
rial, and  that,  as  thus  measured,  no  material  is  ever 
lost,  it  was  supposed  that  in  such  experiments  as  this 
the  substances  involved  underwent  a  mysterious  trans- 
formation ;  the  essence  of  matter,  whatever  it  might 
be,  changing  its  dress,  and  appearing  in  a  new  garb ; 
and  men  reasoned,  "  If  such  transformations  as  these 
are  possible,  why  not  any  others  ?  "  and  hence  centuries 
were  wasted  in  vain  attempts  to  transform  the  baser 
metals  into  gold.  Our  present  convictions  that  such 
transmutation  is  impossible  are  based  on  the  knowl- 
edge we  have  obtained  by  following  to  its  legitimate 
consequences  the  great  principle  established  by  New- 
ton :  when  the  weight  remains,  we  are  persuaded  that 
the  material  remains.  The  weight  of  the  sulphide 
of  iron  is  exactly  equal  to  that  of  the  sulphur  and  iron 
combined.  Hence  we  conclude  that  every  atom  of  the 
iron  and  every  atom  of  the  sulphur  still  remain  in  our 
product,  the  only  difference  being  that,  whereas,  previ- 
ously, the  atoms  of  the  sulphur  were  associated  to- 
gether to  form  molecules  of  sulphur,  and  those  of  the 
iron  to  form  molecules  of  iron,  they  are  now  associated 
with  each  other  to  form  molecules  of  sulphide  of  iron. 
According  to  our  atomic  theory,  then,  in  one  sense 
at  least,  chemical  combination  is  only  a  mixture  of  a 
finer  degree.  If  we  place  on  the  stage  of  a  powerful 
microscope  a  portion  of  the  powder  with  which  we 
have  just  been  experimenting,  we  can  distinguish  the 
grains  of  sulphur  and  those  of  iron,  side  by  side;  and 
so,  according  to  our  theory,  if  we  could  make  micro- 
scopes powerful  enough,  we  should  see  in  the  sulphide 
of  iron  the  atoms  of  its  two  constituents.  But,  al- 
though, in  this  one  respect,  our  modern  chemistry 


122  ELEMENTARY   SUBSTANCES. 

regards  combination  as  merely  a  more  intimate  mix-, 
ture,  yet  it  recognizes  a  very  great  difference  between 
these  two  classes  of  products  indicated  by  a  most  re- 
markable characteristic,  to  which  I  have  next  to  direct 
your  attention. 

Chemical  combination  always  takes  place  in  certain 
definite  proportions,  either  by  weight  or  measure. 
Thus  we  may  mix  together  sulphur  and  iron  in  any 
proportion  wre  choose,  but  when,  on  heating,  combina- 
tion takes  place,  56  grains  of  iron  combine  with  just  32 
grains  of  sulphur ;  and,  if  there  is  an  excess  of  one  or 
the  other  substance,  that  excess  remains  uncombined. 
If  there  is  an  excess  of  sulphur,  there  remains  so  much 
free  sulphur,  which  we  can  dissolve  out  with  sulphide 
of  carbon  ;  and,  if  there  is  an  excess  of  iron,  there  re- 
mains so  much  metallic  iron,  which  we  can  separate 
with  a  magnet.  So  is  it,  also,  in  the  combination  of 
oxygen  with  hydrogen  to  form  water.  Eight  grains  of 
oxygen  combine  with  exactly  one  grain  of  hydrogen, 
and  any  excess  of  either  gas  remains  unchanged,  and 
in  all  cases  of  chemical  combination  and  decomposition 
similar  definite  proportions  are  preserved  between  the 
weight  of  the  several  constituents,  which  unite  to  form 
the  compound,  or  result  from  its  decomposition. 

It  is  an  obvious  explanation  of  these  definite  pro- 
portions that  the  small  particles  or  atoms  between  which 
the  union  is  assumed  to  take  place,  have  a  definite 
weight;  in  other  words,  are  definite  masses  of  mat- 
ter. Now,  the  atomic  theory  supposes,  in  the  com- 
bination of  sulphur  and  iron,  for  example,  that  the  two 
materials  break  up  into  atoms ;  that  an  atom  of  iron 
unites  with  an  atom  of  sulphur  to  form  a  molecule  of 
sulphide  of  iron,  and  that  the  union  takes  place  in  the 
proportion  by  weight  of  56  to  32,  simply  because  these 


COMBINING   PROPORTIONS.  123 

numbers  represent  the  relative  weight  of  the  two  sorts 
of  atoms  (the  atoms  of  the  same  substance  being  all 
alike,  and  all  having  the  same  size  and  weight).  In 
the  case  of  water,  for  reasons  which  will  hereafter  ap- 
pear, it  supposes  that  two  atoms  of  hydrogen  combine 
with  one  atom  of  oxygen  to  form  a  molecule  of  water, 
and,  since  each  atom  of  oxygen  weighs  sixteen  times  as 
much  as  an  atom  of  hydrogen,  the  two  substances  must 
combine  in  the  proportion  of  2  :  16,  or  1  :  8,  as  stated 
above. 

It  will  be  obvious  from  a  moment's  reflection  that 
the  definite  proportions  which  appear  in  these  cases  of 
direct  chemical  union  are  simply  examples  under  the 
general  law  which  governs  all  chemical  processes. 
Chemical  union  is  a  chemical  process  with  definite  fac- 
tors and  a  definite  product.  In  the  first  of  the  exam- 
ples just  cited  sulphur  and  iron  are  the  factors,  and  sul- 
phide of  iron  is  the  product ;  and  in  the  second  example 
hydrogen  gas  and  oxygen  gas  are  the  factors,  while 
water  is  the  product.  In  both  cases  it  is  true — as  in 
the  experiment  we  tried  in  our  fourth  lecture: — not  only 
that  the  weight  of  the  product  exactly  equals  the  sum 
of  the  weights  of  the  factors,  but  also  that  in  each  pro- 
cess the  weights  of  the  several  substances  involved, 
whether  as  factors  or  products,  bear  a  definite  and  con- 
stant relation  to  each  other ;  and,  hence,  that  the  sul- 
phur and  iron,  or  the  oxygen  and  hydrogen,  combine  in 
definite  proportions.  Indeed,  it  was  in  just  these  ex- 
amples of  direct  chemical  union  that  the  law  was  first 
noticed,  and  it  is  this  phase  of  the  law  which  is  usual- 
ly alone  made  prominent  in  text-books  on  chemistry. 
Hence  the  term  combining  proportions,  which  is  fre- 
quently used  in  describing  the  law ;  but  the  law  has  a 
far  wider  range  than  this  term  would  directly  suggest, 


124  ELEMENTARY   SUBSTANCES. 

and  the  larger  scope  can  be  as  easily  apprehended  as 
the  more  restricted.  Hence,  I  have  preferred  to  bring 
before  you  at  the  outset  this  fundamental  principle  of 
chemistry  in  all  its  generality,  so  that  you  would  see 
that  the  combining  proportions — of  which  those  who 
have  previously  studied  chemistry  must  already  have 
heard  so  much — is  simply  one  phase  of  a  more  general 
law,  and  that  this  law  is  in  perfect  harmony  with  our 
conceptions  of  the  constancy  of  the  processes  of  Na- 
ture. 

The  definiteness  of  the  proportions  in  which  sub- 
stances chemically  combine  with  each  other  was  first 
clearly  stated  by  Wenzel  and  Kichter,  in  1777,  and 
the  atomic  theory,  although  itself  as  old  as  philoso- 
phy, was  first  applied  to  the  explanation  of  the  law 
by  the  English  chemist  Dalton,  in  1807.  Subsequent 
discoveries  have  greatly  tended  to  confirm  this  theory, 
but,  before  we  can  appreciate  their  bearing  on  our  sub- 
ject, we  must  endeavor  to  grasp  another  of  the  ele- 
mentary conceptions  of  our  science.  As  in  previous 
cases,  I  shall  not  content  myself  with  stating  the  truth, 
but  endeavor  to  show  how  it  is  deduced  from  observa- 
tion. 

The  study  of  chemistry  has  revealed  a  remarkable 
class  of  substances,  from  no  one  of  which  a  second  sub- 
stance has  ever  been  produced,  by  any  chemical  pro- 
cess, which  weighs  less  than  the  original  substance. 
Let  me  illustrate  what  I  mean  by  a  few  experiments  : 

The  white  powder  which  is  counterpoised  on  the 
pan  of  this  balance  is  called  sulphocyanide  of  mercury, 
and  has  been  used  in  the  preparation  of  a  toy  called 
Pharaoh's  serpent.  You  have  all  probably  seen  the  ex- 
periment, but  perhaps  have  not  observed  the  feature  to 
which  I  wish  to  call  your  attention.  As  in  the  previ- 


PHARAOETS  SERPENT.  125 

ous  experiment,  I  have  made  with  the  powder  a  small 
conical  heap,  and  I  will  now  apply  the  flame  of  a  match 
to  the  apex  of  the  cone.  The  mass  takes  fire  and  burns, 
but,  so  far  from  its  being  consumed,  there  rolls  up  from 
it  a  great  body  of  stuff  whose  singular  shape  suggested 
the  name  of  the  experiment. 

It  is  certainly  a  most  remarkable  chemical  change ; 
for,  from  a  small  amount  of  white  powder,  we  have 
produced  this  great  volume  of  brown  material.  More- 
over, the  conditions  of  the  experiment  are  such  that  it 
is  evident  that  the  material  must  have  been  formed 
from  the  white  powder.  The  only  other  substance 
present  is  the  atmospheric  air,  which,  although  it  plays 
an  important  part  in  the  change,  could  not  have  yield- 
ed this  singular  product.  Notice,  now,  that  the  prod- 
uct, voluminous  as  it  is,  weighs  less  than  the  original 
substance.  This  is  the  feature  of  the  experiment  to 
which  I  wish  especially  to  direct  your  attention,  and 
the  inference  to  be  drawn  from  it  is  obvious.  The  sul- 
phocyanide  of  mercury  has  been  decomposed,  and  the 
material  of  this  brown  mass  was  formerly  a  part  of  the 
material  of  this  substance. 

Allow  me  next  to  recall  to  your  minds  the  experi- 
ments we  made  in  a  previous  lecture  with  sugar.  In 
these  experiments  the  sugar  was  converted  into  charcoal, 
and  the  conditions  were  such  that  the  charcoal  must 
have  come  from  the  sugar,  and  from  nothing  else. 
Now,  since  the  charcoal  weighed  less  than  the  sugar,  it 
was  evident  that  the  material  of  the  charcoal  was  a 
part  of  the  material  of  sugar,  or,  in  other  words,  that 
one  of  the  constituents  of  sugar  is  charcoal.  As  I 
then  stated,  charcoal  was  not  the  only  product  of  those 
chemical  changes.  Water  was  also  produced,  and  un- 
der such  conditions  that  the  material  of  the  water  must 


126  ELEMENTARY  SUBSTANCES. 

have  come  from  the  material  of  sugar,  and  from  that 
alone.  Hence,  we  feel  justified  in  concluding  that  a 
part  of  the  material  of  sugar  is  water ;  and  finding, 
further,  that  the  weight  of  the  charcoal  and  water  to- 
gether is  equal  to  that  of  the  sugar,  we  also  conclude 
that  the  material  of  sugar  consists  of  charcoal  and  wa- 
ter, and  of  these  substances  only. 

So,  also,  in  the  experiment  of  decomposing  water  by 
an  electrical  current,  it  is  evident  that  the  hydrogen  gas 
produced  comes  from  the  water,  and,  as  the  hydrogen 
obtained  weighs  far  less  than  the  water  consumed,  we 
conclude  that  a  part  of  the  material  of  water  is  hydro- 
gen. For  the  same  reasons  we  conclude  that  a  part 
of  the  material  of  water  is  oxygen  ;  and,  lastly,  since 
the  weight  of  the  oxygen  and  hydrogen  together  just 
equals  the  weight  of  the  water,  we  conclude  that  the 
material  of  water  consists  wholly  of  hydrogen  and  oxy- 
gen. Let  me  ask  your  attention  now  to  still  another 
experiment : 

I  have  counterpoised  on  the  pan  of  a  second  bal- 
ance a  few  grammes  of  that  same  finely-pulverized  iron 
which  we  have  already  used  in  this  lecture.  In  this 
condition  metallic  iron  burns  in  the  air  with  the  great- 
est readiness.  We  need  only  touch  the  powder  with 
a  lighted  match  when  a  glow  spreads  through  the  mass 
as  through  tinder.  Notice  that  the  conditions  of  the 
experiment  are  such  that  no  substances  can  concur  in 
the  change  except  iron  and  air.  As  the  result  of  the 
change  a  new  substance  is  produced,  just  as  in  the 
other  cases,  and  this  substance  we  call  oxide  of  iron. 
Is,  then,  this  new  substance  a  part  of  the  material  of 
iron,  in  the  same  sense  that  oxygen  is  a  part  of  the 
material  of  water  ?  The  only  circumstance  which 
points  to  a  different  conclusion  is  what  the  balance 


WHAT  CONSTITUTES  A  CHEMICAL  ELEMENT,          127 

indicates.  The  iron  has  increased  in  weight,  proving 
that  material  has  been  added  to  it,  and  not  taken  from 
it ;  and,  as  you  all  know,  the  iron,  in  burning,  has 
combined  with  the  oxygen  of  the  air.  Oxygen,  then, 
is  the  material  which  has  been  added. 

This  experiment  illustrates  a  most  remarkable  truth 
in  regard  to  the  substance  we  call  iron.  By  various 
chemical  processes  we  can  produce  from  the  metal  hun- 
dreds of  different  substances,  but,  in  all  cases,  the  con- 
ditions of  the  experiment,  and  the  relative  weight  of 
the  products,  prove  that  material  has  been  added  to 
the  iron,  and  not  taken  from  it.  By  no  chemical  pro- 
cess whatever  can  we  obtain  from  iron  a  substance 
weighing  less  than  the  metal  used  in  its  production. 
In  a  word,  we  can  extract  from  iron  nothing  but  iron. 

Now,  there  are  sixty-six  (possibly  seventy-one)  differ- 
ent substances  of  which  this  same  thing  can  be  said. 
From  no  one  ot  these  substances  have  we  been  able  to 
extract  any  material  save  only  the  substance  itself.  We 
are  able  to  convert  them  into  thousands  on  thousands 
of  other  substances ;  but,  in  all  cases,  the  relative 
weight  of  the  products  proves  that  material  has  been 
added  to,  not  taken  from,  the  original  mass.  To  use 
the  ordinary  language  of  science,  we  have  not  been 
able  to  decompose  these  substances,  and  they  are  dis- 
tinguished in  chemistry  as  elementary  substances. 

These  substances  are  frequently  called  chemical  ele- 
ments, but  our  modern  chemistry  does  not  attach  to 
this  term  the  idea  that  these  substances  are  primordial 
principles,  or  self-existing  essences,  out  of  which  the 
universe  has  been  fashioned.  Such  ideas  were  asso- 
ciated with  the  word  element  in  the  old  Greek  philos- 
ophy, and  have  been  frequently  defended  in  modern 
times ;  and,  so  far  as  the  words  element  and  element- 


128  ELEMENTARY   SUBSTANCES. 


List  of  Elementary  Substances. 


Aluminum,    Al, 

27 

Molybdenum,  Mo,     ....     96 

Nickel,             Ni,     59 
Nitrogen,         N,       14 
Norwegium?  Ng,     145.9? 
Osmium,           Os,      199.2 
Oxygen,            O,       16 
Palladium,        Pd,     ....   106.6 
Phosphorus,     P,       31 
Platinum,         Pt,     194.8 
Potassium,        K,      39.1 
Rhodium,         Eh,    104.4 
Rubidium,        Rb,    85.4 
Ruthenium,      Ru,    104.4 
Scandium?        Sc,      44? 
Selenium,         Se,     79.2 
Silicon,              Si,      28 
Silver,               Ag,    108 
Sodium,             Na,    23 
Strontium,        Sr,     87.6 
Sulphur,            S,       32 
Tantalum,         Ta,     ....    182 
Tellurium,         Te,     128 
Terbium?          Tr,     171? 
Thallium,          Tl,     204.1 
Thorium,           Th,     ....   231.4 
Thulium,           Tm,   170 
Tin,                    Sn,     118 
Titanium,          Ti,     50 
Tungsten,          W,     184 
Uranium,           Ur,     240 
Vanadium,        Va,    51.37 
Yttrium,           Y,      91 
Ytterbium,       Yb,    173 
Zinc,                  Zn,     65.2 
Zirconium,        Zr,     89.6 

Antimony,    Sb, 

120 

Arsenic,        As, 

75 

Barium,         Ba, 

137 

210 

Boron,           B, 

11 

Bromine,       Br, 

.  .  .  .     80 

Cadmium,     Cd, 

112 

CtBsium,        Cs, 

133 

Calcium,        Ca, 

40 

Carbon          0, 

.    .       12 

Cerium,         Ce, 

141 

Chlorine        Cl. 

35.5 

Chromium,    Or, 

52.2 

Cobalt,          Co, 

59 

Columbium  Cb, 

94 

CoDDer.         Cu, 

63.3 

159? 

Didymium,    D, 

140 

Erbium         Er, 

166 

Fluorine,       F, 

19 

Gallium,        Ga, 

70 

Glucinum,     Gl, 

13.9 

Gold,             Au,     .  . 

197 

Holmium  2     Ho, 

162? 

Hydrogen      H, 

1 

Indium,          In, 

113.7 

Iodine            I 

.  .     126.8 

Iridium,         Ir, 

192.7 

Iron,              Fe, 

.  ...     56 

Lanthanum,  La, 

.  .    .139 

Lead,            Pb,      .  . 
Lithium         Li        .  . 

.  .    .  206.9 

7 

Magnesium,  Mg, 

24 

Manganese,   Mn, 

55 

Mercury,       Hg, 

.    200 

ary  suggest   such   ideas,  they  are   unfortunate   terms. 
Experimental  science,  which  deals  only  with  legitimate 
deductions  from  the  facts  of  observation,  has  nothing  to 
do  with  any  kind  of  essences  except  those  which  it  can 

CHEMICAL  ELEMENTS  ARE  DEFINITE  SUBSTANCES.     129 

see,  smell,  or  taste.  It  leaves  all  others  to  the  metaphy- 
sicians. It  knows  no  difference  between  elementary 
substances  and  any  other  class  of  substances,  except  the 
one  already  pointed  out.  No  one  can  distinguish  an 
elementary  substance  by  any  external  signs.  Sulphur 
and  charcoal  are  elementary  substances,  chalk  and  flint 
are  compound  substances ;  but.  who  would  know  the 
difference  ?  And,  seventy-five  years  ago,  men  did  not 
know  that  there  was  any  difference.  Modern  chemis- 
try has  shown,  by  a  process  of  reasoning  precisely  simi- 
lar to  that  which  we  have  discussed,  that  out  of  the 
material  of  chalk  we  can  obtain  a  metal  called  calcium, 
and  out  of  flint  a  combustible  substance  called  silicon  ; 
while,  from  the  material  of  charcoal  or  sulphur,  we  can 
educe  no  product  but  the  same  charcoal  or  sulphur 
again.  Hence,  we  say  that  the  first  are  compound  sub- 
stances, and  the  last  elementary ;  but,  were  a  process 
discovered  to-morrow  by  which  a  new  substance  was 
produced  from  the  material  of  sulphur,  we  should  hail 
at  once  the  discovery  of  a  new  element,  and  sulphur 
would  be  banished  forever  from  the  list  of  elementary 
substances.  Yet  the  qualities  of  sulphur  would  not  be 
changed  thereby.  It  would  still  be  used  for  making 
sulphuric  acid  and  bleaching  old  bonnets,  as  if  nothing 
had  happened.  All  this  may  seem  very  trivial,  but 
there  is  no  idea  more  common,  or  of  which  it  is  more 
difficult  to  disabuse  the  mind  of  a  beginner  in  the  study 
of  chemistry,  than  the  notion  that  there  is  something 
peculiar  or  unreal  about  what  is  called  a  chemical  ele- 
ment ;  and  the  conception  that  an  element  is  a  definite 
substance,  like  any  other  substance,  is  usually  the  be- 
ginning of  clear  ideas  on  the  subject.  I  hope  I  have 
been  able  to  make  this  truth  prominent,  and  also  to 
impress  the  further  truth  that  all  our  knowledge  of  the 


130  ELEMENTARY  SUBSTANCES. 

composition  of  matter  is  based  on  the  fundamental 
principle  that  weight  is  the  true  measure  of  quantity 
of  material,  which  is  simply  the  first  postulate  of  the 
law  of  gravitation.  This  great  law  of  Newton  is  thus 
the  basis  of  modern  chemistry  as  much  as  it  is  of  mod- 
ern astronomy. 

"We  are  now  prepared  to  accept  intelligently  the 
following  general  propositions :  1.  That  all  substances 
may  be  resolved  by«  chemical  processes  into  one  or 
more  of  the  seventy-one  elementary  substances ;  2.  That 
all  substances  not  themselves  elementary  may  be  re- 
garded as  formed  by  the  union  of  two  or  more  element- 
ary substances.  Of  course,  the  second  is  merely  the 
reverse  of  the  first,  and  is  implied  by  it ;  but  the  two 
represent  the  two  methods  of  proving  the  constitution 
of  substances,  which  we  have  called  analysis  and  syn- 
thesis. Of  these  the  analytical  proof  alone  is  universally 
possible.  In  by  far  the  larger  number  of  cases,  how- 
ever, we  are  also  able  to  effect  the  synthesis  of  substances 
by  uniting  the  elements  of  which  they  consist,  but 
there  is  still  a  considerable  number  of  substances  which 
have  never  been  produced  in  this  way. 

Having  acquired  the  conception  of  an  elementary 
substance,  and  of  its  chemical  relations,  we  can  now 
give  to  the  law  of  definite  proportions  a  more  precise 
statement.  As  I  have  already  said,  the  law  is  uni- 
versal. It  applies  to  all  kinds  of  chemical  changes, 
and  to  all  classes  of  substances,  elementary  as  well  as 
compound.  But  elementary  substances  are  only  sus- 
ceptible of  that  class  of  changes  we  have  called  syn- 
thetical. They  can  combine  with  each  other,  but  they 
cannot  be  resolved  into  other  substances.  Hence  all 
the  information  in  regard  to  them,  which  the  law,  as 
thus  far  enunciated,  gives  us,  is  that,  when  they  com- 


LAW  OF  MULTIPLE  PROPORTIONS.  131 

bine,  the  union  takes  place  in  definite  proportions  by 
weight  or  volume.  But  this  is  not  all  the  truth.  There 
is  a  law  governing  the  definite  proportions,  and  the 
proportions  of  the  different  elementary  substances 
which  unite  to  form  the  various  known  compounds 
are  so  related  that  it  is  possible  to  find  for  each  ele- 
ment a  number,  such,  that,  in  regard  to  the  several 
numbers,  it  may  be  said  that  the  elements  always  com- 
bine in  the  proportion  by  weight  of  these  numbers 
or  of  some  simple  multiples  of  these  numbers.  This 
supplement  to  the  law  of  definite  proportions  is  known 
as  the  law  of  multiple  proportions  /  but,  if  we  accept 
the  atomic  theory,  both  laws  are  merely  necessary  con- 
sequences of  the  constitution  of  matter  which  this  the- 
ory assumes  to  exist.  Let  us,  in  the  first  place,  under- 
stand fully  the  facts,  and  we  shall  then  be  prepared  to 
consider  their  bearing  on  our  theory. 

In  the  list  of  chemical  elements  above  there  has 
been  placed  against  the  name  of  each  substance  a  num- 
ber which,  for  the  present,  using  a  term  suggested  by 
Davy,  we  will  call  its  proportional  number.  Now,  the 
same  elementary  substances  frequently  combine  with 
each  other  in  several  definite  proportions,  but  these 
proportions,  estimated  by  weight,  are  invariably  those 
of  these  numbers  or  of  their  simple  multiples.  For 
example,  there  are  two  compounds  of  carbon  and  oxy- 
gen, which  contain  the  relative  number  of  parts,  by 
weight,  of  each  element  indicated  below : 

Carbon.  Oxygen. 

Parts  by  weight.  Parts  by  weight. 

Carbonic  oxide,      .....         12  16 

Carbonic  dioxide, 12  32 

There  are  five  compounds  of  nitrogen  and  oxygen 
whose  composition  in  parts,  by  weight,  is  as  follows : 


132  COMBINING  PROPORTIONS. 

Nitrogen.  Oxygen. 

Parts  by  weight.    Parts  by  weight. 

Nitrous  oxide,        ...     28                     16  or  14 :    8 

Nitric  oxide,     ....     14                    16  u  14 :  16 

Dinitric  trioxide,    ...     28                     48  "  14  :  24 

Nitric  dioxide,        ...     14                     32  "  14  :  32 

Dinitric  pentoxide,     .     .     28                     80  "  14 :  40 

Manganese.  Fluorine. 

Parts  by  weight.    Parts  by  weight. 

Manganous  fluoride,     ...     55                   38  =2x19 

Dimanganic  hexafluoride,     .55                   57  =3x19 

Manganic  fluoride, ....     55                  76  =4x19 

Dimanganic  fluoride,   ...    55                114  =  6  x  19 


Examples  like  these  might  be  multiplied  indefinitely, 
and  the  law  holds  not  only  when  two  elements  unite, 
but  also  when  several  unite  in  forming  a  compound. 

There  is  still  another  property  of  these  numbers 
which  must  not  be  passed  unnoticed,  although  it  is  im- 
plied in  what  has  already  been  said.  The  two  num- 
bers, or  their  multiples,  which  express  the  proportions 
in  which  each  of  two  elements  combines  with  a  third, 
express  also  the  proportions  in  which  they  unite  with 
each  other.  Thus,  71  parts  of  chlorine  combine  with 
either  32  parts  of  sulphur  or  with  56  parts  of  iron. 
So,  in  accordance  with  the  law,  56  parts  of  iron  com- 
bine with  32  of  sulphur.  Again,  14  parts  of  nitrogen, 
and  also  381  (=  3  x  127)  parts  of  iodine  combine  with 
3  parts  of  hydrogen,  and  so  14  parts  of  nitrogen  unite 
with  381  of  iodine.  Lastly,  either  16  parts  of  oxygen, 
or  32  parts  of  sulphur,  combine  with  2  parts  of  hydro- 
gen, and  so  32  parts  of  sulphur  combine  with  either 
32  (=  2  x  16)  parts,  or  with  48  (=  3  x  16)  parts  of  oxy- 
gen. In  the  accompanying  table  these  results  are 
given  in  a  tabular  form : 


EXPLAINED  BY  THE  ATOMIC  THEORY. 


133 


32  parts  of  sulphur  combine  with 
56          "      iron  "          " 

56          "        "  "          " 


71  parts  of  chlorine. 

n      "       u 


32 


sulphur. 


14 
3 


nitrogen 
hydrogen 


3  x  127  =  381 
381 

14 


iodine. 


nitrogen. 


16 
32 

32 
32 


oxygen 
sulphur 


"  2 

"  2 

"  2x16  =  32 

"  3  x  16  =  48 


hydrogen. 


oxygen. 


From  the  facts  let  us  pass,  for  a  moment,  to  their 
interpretation,  and  notice  how  they  at  once  suggest  an 
atomic  theory.  To  the  question  which  the  mind  asks, 
"  What  mean  those  definite  weights  ? "  the  suggestion 
comes  at  once,  they  must  mean  definite  masses  of  mat- 
ter ;  they  must  be  the  relative  weights  of  those  little 
masses  we  have  called  atoms.  And  see  what  a  simple 
interpretation  the  atomic  theory  gives  of  this  whole 
class  of  phenomena.  Assume  that  there  are  as  many 
kinds  of  atoms  as  there  are  elementary  substances ;  that 
all  the  atoms  of  the  same  element  have  the  same 
weight,  and  that  the  "  proportional  numbers  "  express 
the  relative  weight  of  the  different  atoms.  Assume 
further  that  combination  consists  merely  in  the  union 
between  atoms,  and  that  chemical  changes  are  deter- 
mined by  their  aggregation,  separation,  or  displace- 
ment, and  we  have  at  once  a  clear  conception  of  the 
manner  by  which  the  remarkable  results  we  have  been 
studying  may  be  produced.  "When  two  elementary 


134  COMBINING  PROPORTIONS. 

substances  combine,  it  must  be  that  a  single  atom,  or 
some  definite  number  of  atoms  of  one,  unite  with  a 
definite  number  of  atoms  of  the  other,  and  therefore 
the  combination  must  take  place  either  in  the  propor- 
tion of  the  relative  weights  of  the  atoms,  or  in  some 
simple  multiple  of  that  proportion.  Moreover,  when  in 
any  chemical  change  a  new  grouping  of  the  atoms  takes 
place,  the  same  relative  proportions  must  be  preserved. 
From  the  conception  of  the  atom  we  naturally  re- 
turn to  that  of  the  molecule,  in  order  to  discuss  the 
relation  between  these  two  quantities,  which  otherwise 
we  should  be  liable  to  confound.  You  remember  the 
physicist's  definition  of  a  molecule  :  "  The  small  par- 
ticles of  a  substance  which  act  as  units."  The  mole- 
cules of  hydrogen  gas  are  the  small,  isolated  masses  of 
hydrogen,  which  move  like  so  many  worlds  through 
the  space  occupied  by  the  gas,  and,  by  striking  against 
the  walls  of  the  inclosure,  produce  the  pressure  which 
the  gas  exerts.  The  molecules  of  water,  in  like  man- 
ner, are  the  small  masses  which  are  driven  apart  by 
heat,  and  become  active  in  the  condition  of  steam. 
The  chemist  looks  at  the  molecule  from  a  somewhat 
different  point  of  view.  To  him  the  small  masses  are 
not  merely  centres  of  forces,  but  they  are  the  particles 
in  which  the  qualities  of  substances  inhere.  They  are 
the  smallest  particles  of  a  substance  which  can  exist  by 
themselves.  So  long  as  the  integrity  of  the  molecule 
is  preserved,  the  substance  is  unchanged,  but,  when  the 
molecules  are  broken  up  or  changed,  new  substances 
are  the  result.  We  can  carry  mechanical  division  no 
further  than  the  molecule,  but,  by  chemical  means,  we 
can  break  up  the  molecules,  and  the  parts  of  the  mole- 
cule thus  brought  to  our  knowledge  are  the  atoms. 
Take,  for  example,  common  salt : 


THE  RELATIVE  WEIGHTS  OF  ATOMS.  135 

The  smallest  particle  of  this  salt  which  has  a  salt 
taste,  and  in  general  retains  the  qualities  of  salt,  is  the 
molecule  of  salt.  This  molecule,  as  we  know  from  the 
specific  gravity  of  the  vapor  of  salt,  weighs  58.5  micro- 
criths.  We  also  know  by  chemical  analysis  that,  in 
every  58.5  parts  of  salt,  there  are  35.5  parts  of  chlo- 
rine and  23  parts  of  sodium0  Hence,  a  molecule  of 
salt  must  contain  35.5  microcriths  of  chlorine  and  23 
microcriths  of  sodium,  and,  in  any  chemical  process  in 
which  chlorine  gas  or  metallic  sodium  is  extracted 
from  salt,  each  molecule  must  be  subdivided  into  these 
two  parts.  Now,  both  chlorine  gas  and  sodium  are 
elementary  substances,  and  our  theory  supposes  that 
the  numbers  35.5  and  23  represent  the  relative  weights 
of  their  atoms.  We,  therefore,  further  conclude  that 
the  molecule  of  salt  is  formed  by  the  union  of  two 
atoms,  one  of  chlorine  and  one  of  sodium. 

In  like  manner,  the  molecules  of  every  compound 
substance  are  aggregates  of  atoms,  of  at  least  two  atoms 
each.  With  the  elementary  substances  it  is  different. 
There  are  many  of  these  whose  molecules  are  never 
subdivided,  and  in  such  cases  the  molecule  and  the 
atom  are  identical,  but  there  are  also  several,  of  which 
the  molecules  can  be  shown  to  consist  of  two  or  more 
atoms.  Thus,  the  molecules  of  phosphorus  probably 
consist  of  four  atoms,  those  of  oxygen  of  two  atoms, 
and  those  of  hydrogen,  nitrogen,  chlorine,  bromine, 
and  iodine,  likewise  of  two. 

Assuming  that  the  molecule  of  hydrogen  gas  con- 
sists of  two  atoms  as  just  stated,  let  us  dwell  on  this 
fact  for  a  moment  as  explaining  our  system  of  estimat- 
ing molecular  weights,  which  must  have  appeared, 
when  stated,  very  arbitrary.  You  remember  that,  ac- 
cording to  the  law  of  Avogadro,  equal  volumes  of  all 


136  COMBINING  PROPORTIONS. 

gases  contain,  under  the  same  conditions,  the  same 
number  of  molecules.  Then,  since  a  given  volume  of 
oxygen  gas  weighs  sixteen  times  as  much  as  the  same 
volume  of  hydrogen  gas,  the  molecule  of  oxygen  must 
weigh  sixteen  times  as  much  as  the  molecule  of  hydro- 
gen ;  and,  if  we  assumed  the  hydrogen-molecule  as  our 
unit  of  molecular  weight,  the  molecule  of  oxygen  would 
weigh  sixteen  of  those  units.  So,  also,  as  nitrogen  gas 
weighs  fourteen  times  as  much  as  hydrogen,  the  nitro- 
gen-molecule would  weigh  fourteen  of  the  hydrogen 
units.  Again,  as  chlorine  gas  weighs  35.5  times  as 
much  as  hydrogen,  a  molecule  of  chlorine  would  weigh 
35.5  of  the  same  units.  But  these  numbers,  16,  14, 
and  35.5,  are  simply  the  specific  gravities  of  the  several 
gases  referred  to  hydrogen ;  so  that,  if  we  took  the 
hydrogen-molecule  as  the  unit,  the  specific  gravity  of 
a  gas  or  vapor  referred  to  hydrogen  would  express  the 
molecular  weight  of  the  substance  in  these  units.  In- 
stead, however,  of  taking  the  hydrogen-molecule  as 
our  unit,  we  selected  the  half-hydrogen  molecule  for 
that  purpose,  and  called  its  weight  a  microcrith,  thus, 
of  course,  doubling  the  numbers  expressing  the  molec- 
ular weights.  Ten  pounds  have  the  same  value  as 
twenty  half-pounds,  and  so  sixteen  hydrogen-molecules 
have  the  same  value  as  thirty-two  microcriths ;  and 
thus  it  is  that,  with  the  system  in  use,  the  molecular 
weight  of  a  substance  is  twice  the  specific  gravity  re- 
ferred to  hydrogen. 

Now,  you  can  understand  the  reason  why  the  half 
hydrogen-molecule  was  selected  as  the  unit  of  molecu- 
lar weight,  and  made  the  microcrith.  It  was  simply 
because  the  half-molecule  is  the  hydrogen  atom.  The 
microcrith  is  simply  the  weight  of  the  hydrogen  atom, 
the  smallest  mass  of  matter  that  has  yet  been  recog- 


WHAT  IS  A  MICROCRITH?  137 

nized  in  science.  The  hydrogen-molecule  consists  of 
two  atoms,  and  therefore  weighs  two  microcriths.  The 
oxygen-molecule  weighs  sixteen  times  as  much  as  the 
hydrogen-molecule,  and  therefore  weighs  thirty-two  mi- 
crocriths. The  specific  gravity  of  carbonic-dioxide  gas 
is  22,  that  is,  it  weighs  twenty-two  times  as  much  as 
hydrogen.  Its  molecule  is  therefore  twenty-two  times 
as  heavy  as  the  hydrogen-molecule,  and,  of  course, 
weighs,  forty-four  microcriths.  Hence,  in  general,  the 
specific  gravity  of  a  gas  referred  to  hydrogen  is  the 
weight  of  the  molecule  as  compared  with  the  hydrogen- 
molecule,  and  twice  the  specific  gravity  of  a  gas  re- 
ferred to  hydrogen  is  the  weight  of  its  molecule  in  hy- 
drogen atoms  or  microcriths. 

But  you  will  ask :  How  do  you  know  that  the  hy- 
drogen-molecule consists  of  two  atoms,  and,  in  gen- 
eral, how  can  you  determine  the  weight  of  the  atom  of 
an  element  ?  This  is  a  very  important  question  for 
our  chemical  philosophy,  and  I  will  endeavor  to  answer 
it  in  the  next  lecture. 


11 


LECTUEE  VII. 

ATOMIC   WEIGHTS   AND   CHEMICAL   SYMBOLS. 

As  I  stated  in  my  last  lecture,  I  am  to  ask  your  at- 
tention at  the  outset  this  evening  to  a  discussion  of  the 
method  by  which  the  chemists  have  succeeded  in  fixing 
what  they  regard  as  the  weights  of  the  atoms  of  the 
several  elements.  This  method  is  based,  in  the  first 
place,  on  the  principle  that  the  molecular  weight  of  a 
substance  can  be  accurately  determined  by  comparing 
its  specific  gravity  in  the  state  of  gas  or  vapor  with  the 
definite  proportions  which  are  invariably  preserved  in 
all  the  chemical  processes  into  which  the  substance  en- 
ters. This  point  has  been  so  fully  explained  that  it  is 
unnecessary  to  enlarge  upon  it  further. 

In  the  second  place,  our  method  is  based  on  the 
principles  of  what  we  call  quantitative  analysis.  I 
have  already  stated  that  the  chemists  have  been  able 
to  analyze  all  known  substances,  and  to  determine  with 
great  accuracy  the  exact  proportions  of  the  several  ele- 
mentary substances  which  are  present  in  each.  The 
methods  by  which  these  results  are  reached  are,  for 
the  most  part,  indirect,  and  frequently  very  compli- 
cated. They  are  described  at  great  length  in  the 
works  on  this  very  important  practical  branch  of  our 
science,  but  it  would  be  impossible  to  give  a  clear  idea 


HOW  SUBSTANCES  ARE  ANALYZED.  139 

of  them  in  this  connection.  It  may  be  well  to  say, 
however,  that,  in  order  to  analyze  a  substance,  it  is  not 
necessary  actually  to  extract  the  several  elementary 
substances  and  weigh  them.  Indeed,  this  can  only 
very  rarely  be  done,  but  we  reach  an  equally  satisfac- 
tory result  by  converting  the  unknown  substance  into 
compounds  whose  composition  has  been  accurately  de- 
termined, and  from  whose  weight  we  can  calculate  the 
weights  of  their  elements. 

For  example,  if  we  wished  to  determine  the  amount 
of  sulphur  in  a  metallic  ore,  we  should  not  attempt  to 
extract  the  sulphur  and  weigh  it.  Indeed,  we  could 
not  do  so  with  any  accuracy ;  but  we  should  act  on  a 
given  weight  of  the  ore,  say  100  grains,  with  appropri- 
ate agents,  and,  by  successive  processes,  convert  all  the 
sulphur  it  contained  into  a  wrhite  powder  called  baric 
sulphate.  Now,  in  accordance  with  the  law  of  definite 
proportions,  the  composition  of  baric  sulphate  is  invari- 
able, and  we  know  the  exact  proportion  of  sulphur  it 
contains.  Hence,  after  weighing  the  white  powder, 
we  can  calculate  the  amount  of  sulphur  in  it,  all  of 
which,  of  course,  came  from  the  100  grains  of  ore. 

Evidently,  this  method  assumes  an  exact  knowledge 
of  the  amount  of  sulphur  in  baric  sulphate,  which  must 
have  been  determined  previously.  This  was,  in  fact, 
found  by  converting  a  weighed  amount  of  sulphur  into 
baric  sulphate,  and,  in  a  similar  way,  most  of  our 
methods  of  analysis  are  based  on  previous  analyses,  in 
which  the  definite  compounds,  whose  composition  wre 
now  assume  is  known,  were  either  resolved  into  ele- 
ments or  were  formed  synthetically  from  the  elements. 

As  the  result  of  such  processes  as  this,  we  have  the 
relative  amounts  of  the  several  elements  present  in  the 
substance  analyzed,  and  it  is  usual  to  state  the  result 


140  ATOMIC  WEIGHTS. 

in  per  cents.     Thus,  the  analyses  of  water,  salt,  and 
sugar,  give  the  results  stated  below  : 


Water. 

Salt. 

Sugar. 

Hydrogen  .  .  . 

11.111 

Sodium  

.   39.32 

Carbon    . 

42.06 

Oxygen  .  . 

88.889 

Chlorine 

.   60.68 

Hydrogen 

6  50 

Oxygen  .  . 

51.44 

100.000 

100.00 

100.00 

Understanding,  then,  that  we  are  in  possession  of 
means  of  determining  accurately  the  weights  of  the 
molecules  of  all  volatile  compounds,  and  also  the  ex- 
act per  cent,  of  any  element  which  each  substance  con- 
tains, we  can  readily  comprehend  the  method  employed 
for  finding  the  weight  of  the  atom.  Let  it  be  the 
weight  of  the  oxygen  atom  which  we  wish  to  deter- 
mine. We  compare  all  the  volatile  compounds  of  oxy- 
gen as  in  the  diagram  (p.  141).  We  take  the  specific 
gravity  of  their  vapors  with  reference  to  hydrogen,  and, 
doubling  the  number  thus  obtained,  we  have  the  molec- 
ular weights  given  in  the  column  under  this  heading. 
The  analyses  of  these  substances  inform  us  what  per 
cent,  of  each  consists  of  oxygen.  Hence,  we  know  how 
much  of  the  molecules  consists  of  this  element.  The 
weight  of  oxygen  in  each  molecule  is  given  in  the  last 
column,  estimated,  of  course,  like  the  molecular  weights, 
in  microcriths.  Having  thus  drawn  up  our  table,  let 
me  call  your  attention  to  two  remarkable  facts  which 
it  reveals. 

Notice,  first,  that  the  smallest  weight  of  oxygen  in 
any  of  these  molecules  is  16  m.c. ;  and,  secondly,  that 
all  the  other  weights  are  simple  multiples  of  this. 

Here,  certainly,  is  a  most  wonderful  fact.  Ke- 
member  that  these  numbers,  which  are  displayed  here 


TABLES. 


141 


Atomic  Weight  of  Oxygen. 


NAMES  OF  COMPOUNDS  OF  OXYGEN. 

Weight  of  mole- 
cub. 

Weight  of  oxygen 
in  molecule. 

Water 

18  m 
28 
30 
46 
74 
44 
46 
64 
60 
80 
104 
146 
208 
263.2 

32     ' 

c. 
i 

16  m 
16     4 
16     4 
16 
16 
32 
32 
32 
48 
48 
48 
48 
64 
64 

32     4 

.c. 
< 

i 

Carbonic  oxide  

Nitric  oxide  

Alcohol  .         ...                         ... 

Ether  

Carbonic  dioxide   

Nitric  dioxide 

Sulphurous  dioxide   

Acetic  acid  

Sulphuric  trioxide. 

Methylic  borate  

Ethylic  borate  

Ethylic  silicate            

Osmic  tetroxide  

Oxygen  g;as.  .  . 

Atomic  Weight  of  Chlorine. 


NAMES  OF  COMPOUNDS  OF  CHLORINE. 

Weight  of  mole- 
cule. 

Weight  of  chlo- 
rine in  molecule. 

Hydrochloric  acid  

36.5  m 
78.5     < 
64.5     ' 
99.       < 
95.       < 
155.2 
181.5 
117.5 
137.5 
154. 
166. 
170. 
359.4 
271.4 
267.8 
237. 

VI. 

c. 
t 

t 

t 
( 
( 
( 
( 

t 
t 
| 

35.5  m 
35.5 
35.5 
VI. 
VI. 
VI. 
106.5 
106.5 
106.5 
142. 
142. 
142. 
177.5 
177.5 
213. 
213. 

VI.       ' 

c. 
c 

Acetylic  chloride  

Ethylic  chloride  

Phosgene  gas  

Dicarbonic  dichloride  

Chromic  oxychloride   

Arsenious  chloride  

Boric  chloride  

Phosphorous  chloride  

Carbonic  tetrachloride  

Dicarbonic  tetrachloride  

Silicic  chloride.  .           

Tantalic  chloride  

Columbic  chloride  

Aluminic  chloride  ...    

Dicarbonic  hexachloride  

Chlorine  gas       ....           ... 

142  ATOMIC   WEIGHTS, 

so  largely,  are  the  results  of  laborious  investigations. 
Each  one  of  them  represents  the  result  of  weeks,  fre- 
quently of  months,  of  labor.  The  molecular  weights 
were  obtained  by  actually  weighing  the  vapor  of  each 
gas,  and  thus  finding  its  specific  gravity ;  the  quan- 
tity of  oxygen  by  analyzing  each  substance,  and  thus 
finding  the  per  cent,  of  oxygen  which  it  contained. 
Remember  that  the  work  has  been  done  at  different 
times,  and  by  many  different  men,  working  wholly  in- 
dependently of  each  other,  and  with  no  view  to  such  a 
result.  Now,  all  this  work  done,  and  the  results  all 
brought  together,  it  appears  that  the  molecule  of  every 
known  oxygen  compound  contains  either  16  micro- 
criths  of  oxygen  or  some  simple  multiple  of  this  quan- 
tity. It  is  impossible  that  this  should  be  a  chance  co- 
incidence. That  invariable  repetition  of  16  microcriths 
must  have  a  meaning,  and  the  only  explanation  we  can 
give  is,  that  it  is  the  weight  of  definite  particles  of  oxy- 
gen, which  we  call  atoms.  In  other  words,  then,  16 
microcriths,  the  smallest  weight  of  oxygen  known  to 
exist  in  any  molecule,  must  be  the  weight  of  the  oxy- 
gen atom.  In  all  those  molecules,  which  contain  16 
m.c.  of  oxygen,  there  is,  then,  1  atom  of  oxygen  ;  in 
those  which  contain  32  m.c.  of  oxygen,  there  are  2 ; 
and,  in  those  which  contain  48  m.c.,  3  atoms,  and  so 
on.  Notice  also,  in  this  connection,  that  the  molecule 
of  oxygen  gas  itself  weighs  32  m.c.,  and  is,  therefore, 
twice  as  heavy  as  the  atom.  In  other  words,  the  mole- 
cule of  oxygen  gas  consists  of  two  atoms,  and  this  is  one 
of  the  cases  referred  to  in  the  last  lecture,  in  which  the 
molecule  of  an  elementary  substance  is  not  the  same  as 
the  atom. 

Take,  now,  another  elementary  substance — chlorine. 
Here  we  have  a  list  of  some  of  the  volatile  compounds 


WEIGHT   OF  THE   CHLORINE  ATOM.  143 

of  this  element.  As  before,  the  molecular  weights 
annexed  were  found  by  means  of  the  known  specific 
gravities  of  the  vapors  of  the  several  substances,  and  the 
weight  of  chlorine  111  each  molecule  was  calculated  from 
the  results  of  oft-repeated  analyses.  Notice  that  the 
smallest  weight  of  chlorine  in  a  molecule  is  35.5  micro- 
criths, and  that  the  other  molecules  have  either  the  same 
weight  or  a  simple  multiple  of  it.  This  number,  35.5, 
appears  here  with  the  same  constancy  as  the  number  16 
in  the  previous  table.  As  before,  this  constancy  cannot 
be  an  accident.  These  35.5  microcriths  of  chlorine 
must  be  definite  masses  of  the  elementary  substance, 
which  retain  their  integrity  under  all  conditions,  and 
are  not  subdivided  in  any  known  chemical  changes,  and 
these  wonderfully  minute  but  definite  masses  are  what 
we  call  the  chlorine  atoms.  The  atoms  of  chlorine,  there- 
fore, weigh  35.5  microcriths.  Hence,  the  molecule  of 
hydrochloric  acid  contains  one  chlorine  atom,  the  mole- 
cule of  phosgene  gas  two  such  atoms,  the  molecule  of 
boric  chloride  three,  that  of  silicic  chloride  four,  and 
that  of  aluminic  chloride  six.  Lastly,  as  in  the  case  of 
oxygen,  the  molecule  of  chlorine  gas  is  twice  as  heavy 
as  the  atom,  or,  as  we  say,  consists  of  two  atoms. 

Consider,  now,  the  facts  in  regard  to  volatile  com- 
pounds of  carbon  as  they  are  shown  in  the  next  dia- 
gram. Here  we  have  a  similar  constancy  in  the  repe- 
tition of  the  number  12.  Twelve  microcriths  is  the 
smallest  quantity  of  carbon  contained  in  the  molecule 
of  any  compound  of  this  element  whose  molecular 
weight  has  been  determined  ;  and  all  molecules  of  car- 
bon compounds,  whose  weight  is  known,  contain  either 
12  microcriths  of  the  elementary  substance,  or  else 
some  whole  multiple  of  12  microcriths.  Again  the 
question  forces  itself  upon  us,  What  means  this  won- 


144 


ATOMIC  WEIGHTS. 


Atomic  Weight  of  Carbon. 


NAMES  OF  COMPOUNDS  OF  CAEBON. 

Weight  of  Mole- 
cule. 

Weight  of  Carbon 
in  Molecule. 

Marsh-gas  

16m. 
28 
60 
74 
88 
207 
98 
152 
120 
136 
148 
169 

c. 

12  m 
24 
36 
48 
60 
72 
84 
96 
108 
120 
132 
144 

c. 

i 
i 

i 

defiant  gas  

Propylic  alcohol  

Ether  

Amylic  alcohol  

Triethylstibine  

Toluol  

Oil  of  wintergreen  

Cuinol  

Oil  of  turpentine  

Amyl  benzol     .    .      .    .            

Diphenylamine  

Atomic  Weight  of  Hydrogen. 


NAMES  OF  COMPOUNDS  OF  HYDROGEN. 

Weight  of  Mole- 
cule. 

Weight  of  Hydro- 
gen in  Molecule. 

Hydrochloric  acid                      

36.5  m. 
81. 
128. 
27. 
18. 
34. 
81.5 
46. 
17. 
34. 
78. 
60. 
28. 
16. 
46. 
74. 

2.       < 

c. 
i 

1m. 
1 
1 
1 
2 
2 
2 
2 
3 
3 
3 
4 
4 
4 
6 
10 

2      ' 

c. 

t 

Hydrobromic  acid  

Hydriodic  acid                 

Hydrocyanic  acid  

"Water  

Hydric  sulphide     

Hydric  selenide.             

Formic  acid  

Ammonia  gas  

Hydric  phosphide  

Hydric  arsenide  

Acetic  acid  

Olefiant  gas  

Marsh-^as    

Alcohol            

Ether  

Hydrogen  gas  

SMALLEST  MASS   OF   MATTER  KNOWN.  145 

derful  constancy  ?  Does  any  one  suspect  that  it  may 
be  a  fiction  of  our  scientific  theorizing—  a  mere  play 
with  numbers  ?  Let  him  only  acquaint  himself  with 
the  facts,  and  he  will  find  how  groundless  his  suspicion 
is.  The  evidence  of  these  facts  is  far  stronger  than 
would  appear  from  our  table.  The  number  of  volatile 
carbon  compounds  is  very  large,  and  our  list  might 
have  been  greatly  extended.  It  must  also  be  constant- 
ly remembered,  as  I  have  said,  that  these  tables  em- 
body the  result  of  a  vast  amount  of  experimental  labor 
—labor,  I  may  add,  without  price,  and  whose  only  ob- 
ject was  the  truth.  Now,  all  this  labor  done,  these 
wonderful  results  appear.  We  must  explain  them ; 
and  the  only  explanation  we  can  give  is,  that  the  mole- 
cules of  these  carbon  compounds  are  formed  of  small 
masses  of  the  elementary  substance  which  weigh  twelve 
microcriths,  and  these  small  masses  are  the  carbon 
atoms. 

Before  leaving  the  subject,  let  me  call  your  atten- 
tion to  one  other  table,  in  which  similar  facts  in  regard 
to  the  volatile  compounds  of  hydrogen  have  been  col- 
lated. Like  the  last,  this  table  might  have  been  great- 
ly extended ;  but  a  sufficient  number  of  facts  have  been 
collected  to  show  that  the  smallest  quantity  of  hydro- 
gen, in  any  molecule,  weighs  one  microcrith,  and  that 
the  quantities  of  this  elementary  substance  in  the  mole- 
cules of  its  various  compounds  are  in  all  cases  whole 
multiples  of  this  small  mass,  which  we  call  the  hydrogen 
atom.  The  hydrogen  atom,  then,  weighs  one  micro- 
crith,  and  the  several  molecules  contain  as  many  hydro- 
gen atoms  as  they  contain  microcriths  of  hydrogen. 
Hence,  the  molecule  of  hydrogen  gas,  which  weighs 
two  microcriths,  consists  of  two  atoms.  The  hydrogen 
atom  is  the  smallest  mass  of  matter  known  to  science, 


146  ATOMIC  WEIGHTS. 

and  I  hope  you  can  now  appreciate  the  reason  why  it 
has  been  chosen  as  the  unit  of  molecular  and  atomic 
weights.  I  also  hope  that  I  have  been  able  to  con- 
vince you  that  it  is  a  definite  mass  of  matter,  and  that 
we  have  as  much  right  to  name  it  a  microcrith  as  to 
call  a  certain  mass  of  metal  a  grain,  or  another  mass  a 
pound. 

In  a  similar  way  the  weights  of  the  atoms  of  the 
other  elementary  substances  have  been  determined  ;  but 
in  this  precise  form  the  method  is  not  universally  ap- 
plicable for  there  are  many  of  the  elementary  substances 
which  do  not  yield  a  sufficient  number  of  volatile  bodies 
to  enable  us,  by  means  of  their  vapor-densities,  to  fix 
the  molecular  weights  of  as  many  of  their  compounds 
as  would  be  required  to  make  our  conclusion  trust- 
worthy. In  such  cases,  however,  we  have  other  methods 
of  finding  the  molecular  weight,  which,  although  not 
so  fundamental  or  so  simple  as  that  based  on  the  spe- 
cific gravity  of  the  vapor,  give  for  the  most  part  satis- 
factory results.  As  we  have  seen,  we  can  always  deduce 
from  the  definite  proportions,  which  a  substance  pre- 
serves in  any  chemical  process  in  which  it  may  be 
involved,  a  value  which  is  either  its  molecular  weight  or 
else  some  simple  multiple  of  this  quantity,  and  in  almost 
all  cases  the  fixed  principles  or  analogies  of  chemical 
science  enable  us  to  decide  which  of  the  possible  values 
is  the  true  weight  sought.  Such  reasoning,  however, 
would  not  be  intelligible  at  the  present  stage  of  our 
study,  but  it  will  appear  conclusive  when  we  have  gained 
that  broader  view  of  chemical  facts  which  it  implies. 

I  trust  we  are  all  now  prepared  to  understand  the 
significance  of  the  numbers  which,  in  the  table  of 
chemical  elements  (on  page  128),  are  associated  with 
the  names  of  the  elementary  substances. 


CONFIRMATION   OF   RESULTS.  147 

These  numbers  represent  the  weights  of  the  several 
atoms  in  microcriths. 

As  I  have  said,  the  idea  that  the  atoms  are  isolated 
masses  of  matter  may  be  a  delusion,  and  so,  as  I  have 
also  intimated,  we  may  doubt  whether  the  magnitudes 
in  optics,  known  as  wave-lengths,  are  the  lengths  of 
actual  ether-waves ;  but,  just  as  these  magnitudes  are 
definite  values,  on  which  we  can  base  calculations 
with  perfect  confidence,  although  the  form  of  the  mag- 
nitude may  not  be  known,  so  the  atomic  weights  are 
invariable  quantities,  whose  relative  values  are  as  well 
established  as  any  data  of  science ;  and,  however  our 
theories  in  regard  to  them  may  change,  they  must  al- 
ways remain  the  fundamental  constants  of  chemistry. 
On  these  data  are  based  all  those  calculations  by  which 
we  predict  the  quantitative  relations  of  chemical  phe- 
nomena, and,  starting  from  the  new  stand-point  which 
they  furnish,  we  shall  now  proceed  to  develop  still 
further  the  philosophy  of  our  science. 

But,  before  we  go  forward,  let  me  call  your  atten- 
tion to  a  very  striking  coincidence,  which  greatly  tends 
to  confirm  the  general  correctness  of  the  results  we 
have  reached : 

You  are  well  aware  that  the  amount  of  Jieat  re- 
quired to  raise  the  temperature  of  the  same  weight  of 
material  to  the  same  degree  differs  very  greatly  for  dif- 
ferent substances.  In  order  to  secure  a  standard  of 
reference,  it  has  been  agreed  to  adopt,  as  the  unit  of 
heat,  the  amount  of  heat-energy  required  to  raise  the 
temperature  of  one  pound  of  water  one  Fahrenheit  de- 
gree, or,  in  the  French  system,  one  kilogramme  of 
water  one  centigrade  degree.  As  water  has  a  greater 
capacity  for  heat  than  any  substance  known  (except 
hydrogen  gas),  it  requires  only  a  fraction  of  a  unit  of 


148 


ATOMIC   WEIGHTS. 


Specific  Heat  of  Elementary  Substances. 


Specific 
Heat. 

Atomic 
Weight. 

Products. 

Lithium  

0.9408 

7. 

6.59 

Sodium   .              .            

0.2934 

23. 

6.75 

Magnesium  

0.2499 

24. 

6.00 

Aluminum    

0.2143 

27. 

5.79 

Phosphorus  

0.1887 

31. 

5.85 

Sulphur  (native)  

0.1776 

32. 

5.68 

Potassium  .                               .    . 

0  1696 

39. 

6.61 

Manganese  

0.1217 

55. 

6.69 

Iron.               .                             ... 

0  1138 

56. 

6.37 

Nickel  

0.1080 

59. 

6.37 

Cobalt                          

0.1073 

59. 

6.33 

Copper  .  . 

0.0951 

63.5 

6.04 

Zinc  .  .  :  

0.0955 

65.2 

6.26 

Arsenic  

0.0814 

75. 

6.11 

Selenium  (metallic).         

0.0761 

79.3 

6.02 

Bromine  (solid)  

0.0843 

80. 

6.75 

Molybdenum  (impure)   

0.0722 

96. 

6.93 

Rhodium  

0.0580 

104.4 

6.07 

Palladium  

0.0593 

106.6 

6.32 

Silver  

0.0570 

108. 

6.16 

Cadmium    

0.0542 

112. 

6.07 

Tin  

0.0562 

118. 

6.63 

Antimony  

0.0508 

120. 

6.09 

Iodine  

0.0541 

127. 

6.87 

Tellurium  

0.0474 

128. 

6.06 

Tungsten  

0.0334 

184. 

6.15 

Gold  

0.0324 

197. 

6.38 

Platinum  . 

0.0324 

194.8 

6.31 

Iridium  

0.0326 

192.7 

6.28 

Osmium  

0.0311 

199.2 

6.20 

Mercury  (solid)  

0.0319 

200. 

6.38 

Thallium  

0.0335 

204. 

6.84 

Lead  

0.0314 

207. 

6.50 

Bismuth  

0.0308 

210. 

6.48 

Boron  (crystallized)                 .... 

0.2500 

11. 

2.75 

Carbon  (diamond)  

0.1469 

12. 

1.76 

Carbon  (graphite)   

0.2008 

12. 

2.41 

Carbon  (wood  charcoal)  

0.2415 

32. 

2.90 

Silicon  (crystallized)..  , 

0.1774 

28. 

4.97 

ATOMS  HAVE  THE  SAME  CAPACITY  FOR  HEAT.    149 

heat  to  raise  the  temperature  of  one  pound  of  any  oth- 
er substance  one  degree.  This  fraction  is  called  the 
specific  heat  of  the  substance,  and  its  value  has  been 
determined  experimentally,  with  great  care,  for  a  very 
large  number  of  substances,  including  most  of  the 
elementary  substances.  In  the  second  column  in  the 
table  on  the  opposite  page  we  have  given  the  specific 
heat  of  more  than  one -half  of  the  elementary  sub- 
stances. We  owe  these  results  to  Regnauit,  and  his  in- 
vestigations on  this  subject  are  among  the  most  impor- 
tant of  the  many  valuable  contributions  to  science  of 
this  eminent  French  physicist.  As  the  specific  heat  of 
a  substance  in  different  states  of  aggregation  often  va- 
ries very  greatly,  only  the  results  obtained  with  the 
elementary  substances  in  the  solid  state  are  here  given, 
and  the  numbers  in  each  case  stand  for  the  fraction  of 
a  unit  of  heat  required  to  raise  the  temperature  of  one 
pound  of  the  solid  one  degree.  The  figures  in  the 
second  column  of  our  table  are  the  atomic  weights  of 
the  elements,  and  those  in  the  third  column  the  prod- 
ucts obtained  by  multiplying  these  weights  by  the  spe- 
cific heat.  Notice  how  constant  this  product  is.  It 
varies  only  between  5.7  and  6.9,  and  there  are  strong 
reasons  for  believing  that  the  variations  depend  on  dif- 
ferences in  the  physical  condition  of  the  elementary 
substances.  We  know  that  this  condition  very  greatly 
influences  the  thermal  relations  of  solid  bodies,  and,  if 
the  substances  could  be  compared  in  precisely  the  same 
state,  it  is  possible  that  the  above  product  would  be 
found  to  be  absolutely  constant,  the  most  probable 
value  being  6.34.  Only  three  solid  elementary  sub- 
stances are  known  the  product  of  whose  atomic  weight 
by  the  specific  heat  does  not  fall  within  the  limits  as- 
signed above,  and  these  are  the  different  forms  of  car- 


150  ATOMIC  WEIGHTS. 

bon,  boron,  and  silicon,  all  elements  remarkable  for  the 
wide  differences  between  the  physical  conditions  in 
which  they  are  known. 

What,  now,  can  be  the  explanation  of  the  remark- 
able law  which  the  table  presents  to  our  notice?  The 
usual  explanation  is,  that  the  atoms  of  the  different  ele- 
ments have  the  same  capacity  for  heat,  and  hence,  that 
masses  of  the  elementary  substances  containing  the 
same  number  of  atoms  must  have  the  same  capacity  for 
heat  when  under  similar  physical  conditions ;  the  con- 
stant product  being  the  amount  of  heat  required  to 
raise  the  temperature  of  such  masses  to  the  same  de- 
gree. If,  for  example,  it  requires  the  same  amount  of 
heat  to  increase  by  one  degree  the  temperature  of  either 
56  rn.c.  of  iron  (one  atom)  or  200  m.c.  of  mercury  (also 
one  atom),  it  will  also  require  equal  amounts  to  raise 
the  temperature  of  56  pounds  of  iron  and  200  pounds 
of  mercury  one  degree;  and  hence  56x0.1138  (the 
specific  heat  of  iron)  must  be  equal  to  200  x  0.0319  (the 
specific  heat  of  mercury). — You  will  remember,  of 
course,  that  the  decimal  in  each  case  represents  the 
fraction  of  a  unit  of  heat  required  to  raise  the  tempera- 
ture of  one  pound  one  degree. 

But,  all  theorizing  apart,  an  agreement  like  this  can- 
not be  the  result  of  accident ;  and,  even  if  we  cannot 
explain  the  law,  the  very  coincidence  gives  us  great 
confidence  in  the  values  of  the  atomic  weights  we  have 
adopted. 

Let  us  now,  for  a  moment,  recapitulate.  All  sub- 
stances are  collections  of  molecules,  and  in  these  mole- 
cules their  qualities  inhere.  What  is  true  of  the  sub- 
stance is  true  of  the  molecule.  The  molecule  is  an  ag- 
gregate of  atoms ;  sometimes  of  atoms  of  the  same  kind, 
as  in  elementary  substances,  sometimes  of  atoms  of 


NOTATION.  151 

different  kinds,  as  in  compound  substances.  The  mole- 
cules are  destructible,  while  the  atoms  are  indestructi- 
ble ;  and  chemical  change  consists  in  the  production  of 
new  molecules  by  the  rearrangement  of  the  atoms  of 
former  ones.  Such,  then,  are  our  conceptions  of  the 
constitution  of  substances,  and  I  next  proceed  to  show 
how  we  are  able  to  represent  this  constitution  by 
means  of  a  most  beautiful  system  of  notation,  with 
which  you  must  be  all  more  or  less  familiar,  under  the 
name  of  chemical  symbols. 

Just  as  in  algebra  letters  are  used  to  represent 
quantities,  so  in  chemistry  we  use  the  initial  letters  of 
the  Latin  name  of  the  elementary  substance  to  repre- 
sent that  mass  of  each  element  we  call  an  atom.  Thus, 
O  represents  one  atom  of  oxygen,  N  one  atom  of  nitro- 
gen, C  one  atom  of  carbon,  Cl  one  atom  of  chlorine, 
Cr  one  atom  of  chromium,  F  one  atom  of  Fluorine,  Fe 
one  atom  of  ferrum  (iron),  S  one  atom  of  sulphur,  Sb 
one  atom  of  stibium  (antimony).  By  using  the  first  let- 
ters of  the  Latin  names,  a  uniformity  has  been  secured 
among  all  nations,  the  convenience  of  which  is  obvious, 
and  it  is  only  in  a  few  cases  that  the  Latin  initial  dif- 
fers from  the  English.  These  symbols  necessarily  rep- 
resent a  definite  weight,  that  is,  the  weight  of  the  atom. 
O  stands  for  16  microcriths  of  oxygen,  C  for  12  micro- 
criths  of  carbon ;  and,  in  each  case,  the  symbol  stands 
for  the  atomic  weight  given  in  our  table  (page  128). 
In  order  to  represent  several  atoms,  we  use  figures 
placed,  like  algebraic  exponents,  above  or  below  the 
symbol.  These  exponents  do  not,  as  in  algebra,  in- 
dicate powers,  but  only  multiples ;  thus,  O2  means  two 
atoms,  or  32  m.c.  of  oxygen,  C6  six  atoms,  or  72  m.c. 
of  carbon,  and  so  on. 

Having  adopted  this  simple  notation  for  the  atom, 


152  CHEMICAL  SYMBOLS. 

we  easily  represent  a  molecule  by  writing  together  the 
symbols  of  the  atoms  of  which  it  consists,  indicating 
the  number  of  each  kind  of  atoms  by  figures,  as  above. 
A  molecule  of  water,  for  example,  consists  of  three 
atoms,  two  of  hydrogen  and  one  of  oxygen.  Hence, 
its  symbol  is  H2O.  This  symbol  shows,  not  only  that 
the  molecule  consists  of  three  atoms,  as  just  stated,  but 
also  that  it  contains  2  m.c.  of  hydrogen  and  16  m.c.  of 
oxygen.  Further,  it  shows  that  the  molecule  of  water 
weighs  18  m.c.  If  we  wish  to  represent  several  mole- 
cules of  water,  we  place  a  figure  before  the  whole  sym- 
bol. Thus,  2H2O  represents  two  molecules  of  water, 
5H2O  five  molecules  of  water,  etc.  Now,  since,  in  all 
chemical  relations,  what  is  true  of  the  molecule  is  true 
of  the  substance,  this  symbol  may  be  regarded  as  the 
symbol  of  water,  and  is  constantly  spoken  of  as  such. 
Again,  a  molecule  of  alcohol  is  known  to  consist  of  two 
atoms  of  carbon,  six  atoms  of  hydrogen,  and  one  of 
oxygen.  Hence,  the  symbol  of  the  molecule  is  C2H6O. 
This  symbol  informs  the  chemist  that  a  molecule  of  al- 
cohol contains  2  atoms  or  24  m.c.  of  carbon,  6  atoms 
or  6  m.c.  of  hydrogen,  and  1  atom  or  1G  m.c.  of  oxy- 
gen. It  also  shows  that  the  total  weight  of  the  mole- 
cule is  46  m.c.  Several  molecules  of  alcohol  are  in- 
dicated by  the  use  of  coefficients,  as  before — thus 
3C2H6O,  etc.  This  is  the  whole  of  the  system,  and  you 
see  how  beautiful  and  simple  it  is.  The  single  letters 
stand  for  atoms,  and  the  terms  formed  by  the  grouping 
of  the  letters  stand  for  molecules,  and  the  very  possi- 
bility of  the  system  is  in  itself  a  very  strong  proof  that 
molecules  and  atoms  really  exist. 

Before  proceeding  to  show  how  admirably  this 
system  is  suited  to  express  chemical  changes,  let  me 
ask  yo\ir  attention  for  a  moment  to  the  nature  of  the 


SYMBOL  OF  ALCOHOL,  HOW  DETERMINED,    15e 

evidence  by  which  the  symbol  of  a  substance  is  fixed  ; 
for,  although  this  evidence  is  precisely  of  the  same  kind 
as  that  on  which  the  atomic  weights  of  the  elementary 
substances  rest,  yet  the  principles  involved  are  so  im- 
portant that  a  brief  restatement  of  the  evidence,  as  it 
bears  on  the  present  problem,  seems  almost  necessary 
for  a  clear  understanding  of  the  subject.  The  question 
is  this :  What  is  your  proof  that  the  symbol  of  alcohol, 
for  example,  is  C2H6O,  or,  in  other  words,  that  this 
symbol  represents  the  constitution  of  a  molecule  of  al- 
cohol ?  The  evidence  is — 

1.  We  know  by  experiment  (page  81)  that  the  spe- 
cific gravity  of  alcohol-vapor   referred  to  hydrogen  is 
23.     Hence,  since,   by  Avogadro's   law   alcohol- vapor 
and  hydrogen  gas  have  in  the  same  volume  the  same 
number  of  molecules,  the  molecule  of  alcohol  is  twenty- 
three  times  as  heavy  as  the  molecule  of  hydrogen  gas ; 
and,  further,  since  by  assumption  the  hydrogen-mole- 
cule weighs  2  m.c.,  the  alcohol-molecule  weighs  46  m.c. 

2.  We  have  analyzed  alcohol,  and  know  that  it  has 
the  following  composition : 

Analysis  of  Alcohol. 

Per  cent.         Composition  of 
molecuie. 

Carbon 52.18  24  m.c. 

Hydrogen 13.04  6     " 

Oxygen , 34.78  16     " 

100.00  46     " 

Hence,  of  the  molecule  of  alcohol  52T1^  per  cent., 
or  24  parts  in  46,  consist  of  carbon,  13-j-^-  per  cent., 
or  6  parts  in  46,  consist  of  hydrogen,  find  34T7Q-8Q-,  or  16 
parts  in  46,  consist  of  oxygen.  The  whole  adds  up,  as 
you  see,  46,  showing  that  we  have  done  our  sum  cor- 
rectly. 

12 


154  CHEMICAL  SYMBOLS. 

Analysis,  then,  proves  that,  of  the  molecule  of  alco- 
hol weighing  46  m.c.,  24  m.c.  are  carbon,  6  m.c.  are 
hydrogen,  and  16  m.c.  are  oxygen.  But  the  weight  of 
an  atom  of  carbon  is  12  m.c.,  hence  the  molecule  con- 
tains two  atoms  of  carbon,  or  C2  ;  the  weight  of  an  atom 
of  hydrogen  is  1  m.c.,  hence  the  molecule  contains  6 
atoms  of  hydrogen,  or  H6  ;  the  weight  of  the  oxygen 
atom  is  16  m.c.,  hence  the  molecule  contains  one  atom 
of  oxygen,  or  O,  and  the  symbol  is  C2H6O. 

Again,  why  is  the  symbol  of  water  H2O?  1.  The 
specific  gravity  of  steam  referred  to  hydrogen  gas  is  9, 
hence  the  weight  of  a  molecule  of  water  in  microcriths 
is  18.  2.  Analysis  shows  that  water  has  the  following 
composition  in  100  parts  : 

Analysis  of  Water. 


Hydrogen  .......................   11.11  2  m.c. 

Oxygen  .........................  88.89  16     " 

100.UO  18     " 

We  know,  then,  that,  of  the  molecule  weighing  18 
m.c.  of  water,  ll^V  per  cent.,  or  2  m.c.,  consist  of 
hydrogen,  and  88^  per  cent.,  or  16  m.c.,  consist  of 
oxygen.  But  2  m.c.  of  hydrogen  equal  2  atoms,  or 
H2,  and  16  m.c.  of  oxygen  1  atom,  or  O.  Hence,  the 
symbol  is  H2O. 

You  see  how  simple  is  the  reasoning  and  how  defi- 
nite the  result  ;  and,  unless  our  whole  theory  in  regard 
to  molecules  and  atoms  is  in  error,  there  is  no  more 
doubt  that  the  symbol  of  water  should  be  written  H2O, 
than  that  this  familiar  liquid  consists  of  oxygen  and 
hydrogen  gas. 

But  many  of  my  audience  will  remember  that, 
when  they  studied  chemistry,  the  symbol  of  water  was 


WHY   IS   H20   THE   SYMBOL   OF   WATER?  155 

HO,  and  will  ask,  Why  this  change  ?  I  answer :  This 
difference  is  of  a  type  with  the  whole  difference  be- 
tween the  old  and  the  new  schools  of  chemistry.  In- 
deed, the  two  symbols  may  be  regarded  as  the  shibbo- 
leths of  the  two  systems.  In  the  old  system,  the  sym- 
bols simply  stood  for  proportions,  and  nothing  else. 
The  symbol  H  meant  1  part  by  weight  of  hydrogen, 
and  O  8  parts  by  weight  of  oxygen  :  and  HO  meant 
a  compound,  in  which  the  two  elements  were  com- 
bined in  the  proportions  o/  1  to  8,  which  is  as  true  of 
water  now  as  it  was  then.  In  the  old  system,  the  spe- 
cial form  of  the  symbol,  whether  H2O,  HO,  or  HO2, 
had  no  significance,  for  this  was  determined  by  the  ar- 
bitrary values  given  to  the  letters.  There  is  a  second 
compound  of  hydrogen  and  oxygen  called  hydric  perox- 
ide, in  which  the  elements  are  combined  in  the  propor- 
tion of  1  of  hydrogen  to  16  of  oxygen ;  and,  had  the 
chemists  of  the  old  school  assigned  to  the  symbol  O  the 
value  16  instead  of  8,  then  the  symbol  of  hydric  per- 
oxide would  have  been  written  HO,  and  that  of  water 
H2O ;  and  the  only  reason  usually  given  for  making  O 
represent  8  parts  of  oxygen  instead  of  16  was,  that 
water,  being  very  widely  diffused  in  Nature,  and  the 
most  stable  compound  of  the  two,  ought  to  be  repre- 
sented by  the  simplest  symbol ;  or,  in  other  words,  that 
the  ratio  between  the  quantities  of  oxygen  and  hydro- 
gen, which  it  contains,  ought  to  be  taken  as  the  type 
ratio  between  these  elements. 

This  reasoning  was  as  unsatisfactory  as  it  has 
proved  to  bo  unsound.  It  might  justly  have  been  said 
that  the  system,  although  artificial,  was  consistent  in 
itself,  and  that  it  better  suited  the  requirements  of  the 
system  to  assign  to  oxygen  the  proportional  number  8, 
than  to  select  a  multiple  of  that  number.  Indeed,  this 


156  CHEMICAL   SYMBOLS. 

was  the  light  in  which  the  whole  scale  of  proportional 
numbers  was  regarded  by  a  large  majority  of  the  stu- 
dents of  chemistry  during  the  first  half  of  this  cen- 
tury ;  and  it  is  only  necessary  to  state  that  the  German 
chemists,  following  the  lead  of  Berzelius,  used  for  years 
a  scale  in  which  oxygen  was  taken  as  100,  in  order  to 
show  how  purely  arbitrary  the  actual  numbers  were 
considered  to  be.  The  only  truth  that  the  numbers 
were  believed  to  represent  was  the  law  of  definite  and 
multiple  proportion ;  and,  so  long  as  the  true  propor- 
tions were  preserved,  any  scale  of  numbers  might  be 
used  which  suited  the  experimenter's  fancy. 

It  is,  however,  perfectly  true  that,  in  selecting  one 
of  several  multiples,  which  might  be  used  for  a  given 
element  in  a  given  scale,  the  decision  of  the  chemist 
was  not  unfrequently  influenced  by  the  very  ideas 
which  now  form  the  basis  of  our  modern  science ;  as  is 
shown  by  the  fact  that  the  proportional  numbers  of 
Davy  and  Berzelius  were  called  chemical  equivalents 
by  Wollaston,  and  atomic  weights  by  Dalton  and  his 
pupils.  But,  then,  the  truths,  which  these  terms  now 
imply,  were  never  fully  conceived  or  consistently  car- 
ried out.  The  atomic  weights  of  the  new  system  are 
the  weights  of  real  quantities  of  matter,  the  combining 
numbers  of  the  old  system  were  certain  empirical  pro- 
portions. So  is  it  in  other  particulars,  and  the  differ- 
ence between  the  new  school  and  the  old  is  really  the 
difference  between  clear  and  misty  conceptions. 

Our  modern  science  is  a  philosophical  system,  based 
on  ideas  distinctly  stated  and  consistently  developed. 
The  chemists  of  the  old  school  can  hardly  be  said  to 
have  had  a  philosophy,  but  they  had  an  admirable  no- 
menclature, which  was  almost  as  good  as  a  philosophy, 
and  served  to  classify  the  facts  while  the  fundamental 


CHARACTERISTICS   OF  THE   NEW  SCHOOL.  157 

principles  of  the  science  were  being  slowly  developed. 
It  was,  of  course,  to  be  expected  that  the  fundamental 
ideas  of  our  science  should  be  conceived  separately  and 
at  first  only  imperfectly  ;  and  it  was  not  until  clear  and 
definite  conceptions  had  been  reached,  and  the  rela- 
tions of  the  several  ideas  clearly  understood,  that  a 
philosophy  of  chemistry  was  possible.  Of  course,  we 
are  far  from  believing  that  the  ideas,  now  prevailing, 
are  necessarily  true,  and  it  is  perhaps  to  be  expected 
that  our  modern  school  will  share  the  same  fate  as  that 
which  preceded  it ;  but  we  do  believe  that  the  coming 
system,  whatever  it  may  be,  will  be  based  on  equally 
clear  conceptions,  and  that,  in  attempting  to  clarity  our 
ideas  and  realize  our  conceptions,  we  are  following  the 
right  path,  and  making  the  only  satisfactory  progress. 

Before  closing  the  lecture,  it  only  remains  for  me  to 
show  how  the  system  of  notation  I  have  described  may 
be  used  to  express  chemical  changes,  and  I  can  best 
illustrate  this  use  by  applying  it  in  a  practical  exam- 
ple. The  experiment  I  have  selected  for  the  purpose 
must  be  familiar  to  every  one  in  some  form  or  other. 

In  the  first  place,  we  have  in  this  large  glass  vessel 
a  white,  pulverulent  solid,  familiarly  called  soda.  The 
chemists  call  it  sodic  carbonate.  It  consists  of  mole- 
cules, which  are  each  formed  of  six  atoms,  two  of  a 
metal  called  sodium,  one  of  carbon,  and  three  of  oxy- 
gen. Hence,  the  symbol  is  Na2CO3.  In  the  second 
place,  we  have  in  this  pitcher  a  liquid  well  known  in 
commerce  under  the  name  of  muriatic  acid.  It  is  a 
solution  in  water  of  a  compound  which  is  called  in 
chemistry  hydrochloric  acid.  Hydrochloric  acid  itself, 
as  I  shall  show  you  at  the  next  lecture,  is  a  gas 
18J  times  as  heavy  as  hydrogen  ;  hence  its  molecular 
weight  is  36^ — and  its  molecules,  as  is  well  known,  con- 


158  CHEMICAL   SYMBOLS. 

sist  of  one  atom  of  chlorine  and  one  of  hydrogen. 
Its  symbol  is  then  HC1 — and  the  condition  of  aqueous 
solution  we  may  express  by  the  addition  of  the  letters 
Aq,  the  initial  of  aqua,  the  Latin  name  of  water — thus : 
IlCl  +  Aq. 

On  pouring  the  acid  upon  the  soda,  there  is  at  once 
a  violent  etiervescence ;  and  a  large  quantity  of  gas  is 
evolved,  which  will  soon  fill  the  glass  jar.  The  old 
substances  disappear,  and  new  substances  are  formed. 
This,  then,  is  a  chemical  process,  and  such  a  process,  in 
the  technical  language  of  chemistry,  is  usually  called  a 
reaction  /  and  as  hitherto  we  have  spoken  of  the  factors 
and  products  of  a  chemical  process,  so  hereafter  we  shall 
use  the  same  terms  in  describing  chemical  reactions. 

In  the  present  example,  the  factors  are  sodic  car- 
bonate, hydrochloric  acid,  and  water.  What  are  the 
products  I 

First  of  all,  we  have  a  large  volume  of  colorless 
gas,  and  not  only  a  large  volume,  but  also  a  very  con- 
siderable weight,  since,  for  a  gas,  it  is  quite  a  heavy 
substance.  In  old  times  this  product  of  the  process  was 
wholly  overlooked ;  but  I  can  easily  prove  to  you  that 
there  is  a  no  inconsiderable  amount  of  material  in  the 
upper  part  of  this  glass  vessel,  although  in  an  invisible 
condition.  First,  by  lowering  a  lighted  candle  into  the 
jar,  I  can  show  that  the  air  has  been  displaced  by  a 
medium  in  which  the  candle  will  not  burn.  In  the 
second  place,  by  dipping  out  some  of  the  gas  and  pour- 
ing it  into  this  paper  bucket,  I  can  make  evident  that 
its  weight  is  appreciable:  You  notice  that  the  end  of 
the  balance-beam  to  which  the  bucket  is  suspended 
immediately  falls ;  and  see,  also,  how  these  candles  are 
extinguished,  as  the  heavy  gas  from  my  dipper  flows 
down  on  the  flames.  Lastly,  by  repeating  the  experi- 


USED   TO   EXPLAIN   A    FAMILIAR   PROCESS.  159 

ment  on  a  smaller  scale  in  front  of  the  lantern,  and 
projecting  the  image  of  the  small  glass  vessel,  we  here 
use,  on  the  screen,  I  can  make  the  current  of  gas  visible 
as  it  flows  over  the  lip. 

This  aeriform  material  is  now  called  in  chemistry 
carbonic  dioxide,  but  you  are  more  familiar  with  it  un- 
der the  old  name  of  carbonic  acid.  It  is  the  chief  prod- 
uct of  the  burning  of  coal  and  wood ;  and,  when  you 
are  told  that  every  ton  of  coal  burned  yields  3f  tons  of 
this  gas,  you  can  conceive  what  immense  floods  are  be- 
ing constantly  poured  into  the  atmosphere  from  the 
throats  of  our  chimneys.  It  is  also  being  continually 
formed,  and  in  still  greater  amounts,  by  the  processes 
of  respiration,  fermentation,  and  decay.  Although  fa- 
miliarly known  only  in  the  state  of  gas,  it  can  readily 
be  reduced  by  pressure  and  cold  to  the  liquid  condition  ; 
and,  when  in  this  condition,  is  easily  frozen,  forming  a 
transparent  solid  like  ice,  or  a  loose,  flocculent  material 
like  snow,  under  different  conditions.  It  is  a  com- 
pound simply  of  carbon  and  oxygen,  and  no  fact  of 
chemistry  is  better  established  than  that  every  mole- 
cule of  this  gas  consists  of  one  atom  of  carbon  and  two 
atoms  of  oxygen.  Hence  its  symbol  is  CO2. 

The  presence  of  the  other  products  formed  in  our 
experiment  I  cannot  make  so  readily  evident  to  you, 
although  they  are  really  far  more  tangible  than  this 
gas.  One  of  them  is  water,  which  at  once  mingles 
with  the  large  body  of  water  used  in  the  experiment. 
The  other  is  common  salt.  This  dissolves,  as  it  forms, 
in  the  water  present ;  but,  after  the  reaction  is  ended, 
it  can  easily  be  isolated  by  evaporating  the  brine.  We 
will  start  the  process,  so  that  any  one  who  is  skeptical 
can  satisfy  himself,  by  tasting  the  residue,  that  common 
salt  has  been  really  formed. 


160  CHEMICAL  SYMBOLS. 

Common  salt  is  composed  of  a  metal,  sodium,  and 
chlorine  gas.  Its  molecules  are  known  to  consist,  each 
of  an  atom  of  sodium  and  an  atom  of  chlorine.  Hence 
its  symbol  is  NaCl. 

Let  us  now  write  the  factors  of  this  reaction  oppo- 
site to  the  products,  so  that  we  can  compare  them  : 

Na2CO3          HC1  NaCl  H20         CO2. 

Sodic  Hydrochloric  Sodic  Chloride,  or     Water.         Carbonic 

Carbonate.  Acid.  Common  Salt.  Dioxide. 

Now,  let  me  remind  you  of  a  simple  principle, 
which  we  must  apply  in  interpreting  this  reaction. 
No  material  can  be  lost.  These  atoms  are  indestructi- 
ble, so  far  as  we  know.  If,  then,  we  have  here  all  the 
factors  and  all  the  products  (and  there  can  be  no  doubt 
whatever  on  this  point),  there  must  be  just  as  many 
atoms  of  each  element  in  the  products  as  there  are  in 
the  factors,  and  vice  versa.  Now,  there  are  two  atoms 
of  sodium  in  the  molecule  of  sodic  carbonate.  Hence 
there  must  be  two  atoms  of  the  same  element  in  the 
products,  and  we  must  therefore  write  2NaCl.  The 
molecule  of  water  in  the  products  has  two  atoms  of 
hydrogen  ;  hence  we  must  write  2HC1  among  the  fac- 
tors. Thus  amended,  our  reaction  becomes  : 

JSTa2CO3  +  2HC1  =  2NaCl  +  H2O  +  C02. 

Now,  since  the  quantity  of  material  represented 
among  the  products  exactly  equals  that  represented 
among  the  factors,  we  may  very  properly  employ  the 
equation-sign  of  algebra  to  separate  the  two  members 
of  our  reaction  ;  and,  further,  it  becomes  equally  nat- 
ural to  separate  the  several  terms  by  the  plus  sign. 
When,  now,  we  study  the  chemical  change,  as  thus 
written  out  for  our  inspection,  we  see  that,  in  the  pro- 
cess, each  molecule  of  sodic  carbonate  is  acted  upon  by 
two  molecules  of  hydrochloric  acid.  The  two  atoms 


REPRESENT  CHEMICAL  CHANGES.        161 

of  sodium  (Na2)  from  the  molecule  of  sodic  carbonate 
(Na2CO3)  unite  each  with  an  atom  of  chlorine  (Cl) 
from  the  two  molecules  of  hydrochloric  acid  (2HC1), 
and  there  are  thus  formed  two  molecules  of  common 
salt  (2NaCl).  Meanwhile,  the  original  molecules  hav- 
ing been  broken  up,  the  other  atoms  group  themselves 
together  to  form  a  molecule  of  water  (H2O)  and  a  mole- 
cule of  carbonic  dioxide  (CO2).  In  a  word,  the  chemi- 
cal change  consists  in  the  breaking  up  of  the  old  mole- 
cules and  the  rearrangement  of  the  atoms  to  form 
others,  and  you  will  notice  how  perfectly  our  system  of 
symbols  enables  us  to  follow  the  steps  of  the  process. 

In  saying  that  this  equation  represents  the  pro- 
cess, we  assume  the  truth  of  the  principle,  already  so 
often  reiterated,  that  what  is  true  of  the  molecules  is 
true  of  the  substances.  Our  equation  merely  repre- 
sents the  reaction  between  one  molecule  of  sodic  car- 
bonate and  two  of  hydrochloric  acid.  Of  course,  there 
were  billions  on  billions  of  molecules  in  our  glass  jar, 
but  then  the  action  here  represented  was  simply  so 
many  billion  of  times  repeated. 

There  is  only  one  other  point  in  connection  with 
this  experiment  to  which  I  wish  to  call  your  special  at- 
tention before  closing  the  lecture.  "We  used  a  great 
deal  of  water  in  the  process,  and  the  experiment  would 
not  have  succeeded  without  it.  Now,  what  part  does 
the  water  play?  An  essential  part — and  this  point  has 
a  most  important  bearing  on  our  theory  of  molecules. 

The  reaction  we  have  been  studying  takes  place,  as 
we  have  said,  between  molecules.  But,  in  order  that 
the  molecules  of  the  one  body  should  act  on  those  of  the 
other,  it  is  obviously  necessary  that  they  should  have  a 
certain  freedom  of  motion.  If  the  molecules  had  been 
rigidly  fixed  in  the  material  of  the  two  substances,  it 


162  CHEMICAL  SYMBOLS. 

would  obviously  have  been  impossible  for  them  to  mar- 
shal themselves  in  the  manner  we  have  described,  two 
of  one  substance  associating  with  one  of  the  other  in 
the  resulting  chemical  process.  Now,  in  a  solid  body, 
the  molecules  are  to  a  great  extent  fixed,  and  hence  no 
chemical  action  is  possible  between  such  substances, 
except  to  a  limited  extent.  There  are,  in  general,  two 
ways  by  which  the  required  freedom  of  motion  can  be 
obtained :  One  is  to  convert  the  substance  into  vapor, 
when,  as  we  have  seen,  the  molecules  become  com- 
pletely isolated,  and  move  with  great  velocity  through 
space,  their  motion  being  only  limited  by  the  walls  of 
the  containing  vessel ;  but  this  method  is  only  appli- 
cable to  volatile  bodies.  The  second  method  is  to  dis- 
solve the  solid  in  some  solvent,  when  the  molecules,  as 
before,  become  isolated,  and  move  freely  through  the 
mass  of  the  liquid.  The  last  is  the  method  generally 
used,  and  water,  being  such  a  universal  solvent,  is  the 
common  vehicle  employed  to  bring  substances  together, 
and  for  that  reason  it  enters  into  a  very  great  number 
of  chemical  changes.  Such  was  its  office  in  the  process 
we  have  been  studying.  We  dissolved  both  the  sodic 
carbonate  and  the  hydrochloric  acid  in  water,  in  order 
that  their  molecules  might  readily  coalesce.  An  experi- 
ment will  enforce  the  principle  I  have  been  enunciating: 
There  are  a  great  many  substances  which  will  act 
on  sodic  carbonate  like  hydrochloric  acid ;  for  example, 
almost  all  the  so-called  acids  or  acid  salts,  and,  among 
others,  that  white  solid  with  which  you  are  familiar  un- 
der the  name  of  cream -of-tartar.  Here  we  have  cream- 
of-tartar  and  sodic  carbonate,  both  in  fine  powder,  and 
we  have  been  carefully  mixing  them  together  in  this 
mortar.  You  see,  there  is  no  action  whatever ;  and,  in 
a  dry  place,  we  can  keep  the  mixture  indefinitely  with- 


CONDITION  OF  SOLUTION  REPRESENTED.  163 

out  change.  If,  however  (placing  the  mixture  in  this 
glass  vessel),  we  pour  water  over  it,  we  have  at  once  a 
brisk  effervescence,  and  carbonic  dioxide  is  evolved  as 
before.  It  required  the  water  to  bring  the  molecules 
together. 

"  Since,  then,  the  water  plays  such  an  important  part 
in  the  reaction,  I  prefer  to  indicate  its  presence,  arid 
this  may  be  done  by  using  the  symbol  Aq.  as  previously 
described. 

(Na2CO3  4-  2H01  +  Aq.)  =  (2NaCl  +  H2O  +  Aq.)  +  CO7. 
Solution  of  Sodic  Carbonate  Solution  of  Common  Salt. 

and  Hydrochloric  Acid. 

This  indicates  not  only  that  both  of  the  factors  are 
in  solution,  but  also  that  we  have,  as  one  of  the  prod- 
ucts, a  solution  of  common  salt.  That  the  second  prod- 
uct, carbonic  dioxide,  is  a  gas,  I  sometimes  indicate  by 
a  line  drawn  over  the  symbol,  as  above. 

The  second  reaction  is  equally  simple,  but  cream- 
of-tartar  has  a  vastly  more  complex  molecule  than  IIC1. 
Its  symbol  is  HKC4H4O6,  that  is,  each  molecule  con- 
sists of  four  atoms  of  carbon,  six  atoms  of  oxygen,  one 
atom  of  potassium,  and  five  atoms  of  hydrogen.  I 
write  one  of  the  atoms  of  hydrogen  apart  from  the 
rest,  because  it  has  a  very  different  relation  to  the 
molecule  —  a  relation  which  I  shall  hereafter  explain. 
The  reaction  would  be  written  thus  : 


(Na2C03  +  2HKC4H4Oo  +  Aq.)  = 


(2NaKC4H406  +  H2O  +  Aq.)  +  CO2. 

Solution  of  Rochelle  Salts. 

With  this  reaction  many  of  my  audience  must  be 
familiar,  as  a  mode  of  raising  dough  in  the  process  of 
making  bread.  The  first  member  of  the  equation  in- 
dicates that  the  two  substances  are  used  in  solution. 
There  is  formed,  as  the  product  of  the  reaction,  be- 
sides the  carbonic  dioxide  gas,  which  puffs  up  the 


164  CHEMICAL  SYMBOLS. 

dough,  the  solution  of  a  salt,  whose  molecule  has  the 
complex  constitution  I  have  indicated,  and  which  is  a 
well-known  medicine  under  the  name  of  Rochelle-salts. 
When  soda  and  cream-of-tartar  are  used  in  making 
bread,  this  salt  remains  in  the  loaf.  The  amount 
formed  is  too  small  to  be  injurious,  but  I  cannot  but 
think,  although  it  may  be  a  prejudice,  that  chemicals 
had  better  be  kept  out  of  our  daily  bread. 


LECTURE  VIII. 

CHEMICAL     BE  ACTIONS. 

To  master  the  symbolical  language  of  chemistry,  so 
as  to  understand  fully  what  it  expresses,  is  a  great  step 
toward  mastering  the  science ;  and  so  important  is  this 
part  of  my  subject  that  I  propose  to  occupy  the  hour 
this  evening  with  a  number  of  illustrations  of  the  use 
of  symbols  for  expressing  chemical  changes. 

First,  I  will  recur  to  the  experiment  of  the  last 
lecture,  for  we  have  not  yet  learned  all  that  it  is  cal- 
culated to  teach. 

Let  us  again  write  on  the  black-board  the  symbols 
which  represent  the  chemical  process : 

(Na2C03  +  2HC1  -f  Aq.)  =  (2NaCl  +  H2O  +  Aq.)  +  COT. 

Sodic  Hydrochloric  Common  Water.  Carbonic 

Carbonate.  Acid.  Salt.  Dioxide  Gas. 

We  bring  together  a  solution  of  sodic  carbonate 
and  hydrochloric  acid ;  and  there  are  formed  as  prod- 
ucts a  solution  of  common  salt,  water,  and  carbonic 
dioxide  gas.  I  need  not  refer  again  to  the  circum- 
stance that  the  state  of  solution  is  an  essential  condi- 
tion of  the  change,  for  this  point  was  fully  discussed 
at  the  time ;  but,  before  we  pass  on  to  another  experi- 
ment, I  wish  to  call  your  attention  to  the  fact  that  the 
several  terms  in  this  equation  stand  for  absolutely  defi- 


166  CHEMICAL  REACTIONS. 

nite  weights  of  the  quantities  they  represent.  Each 
symbol  stands  for  .the  known  weights  of  the  atoms 
which  are  tabulated  in  this  diagram  (table,  page  128), 
and  the  weights  of  the  molecules,  which  the  several 
terms  represent,  are  found  by  simply  adding  up  the 
weights  of  the  several  atoms  of  which  they  consist. 
When  the  substance  is  capable  of  existing  in  the  aeri- 
form condition,  its  molecular  weight  can  be  found,  as  I 
have  shown,  from  its  specific  gravity ;  but  these  sym- 
bols assume  that  either  by  this  or  by  some  other  method 
the  constitution  of  the  molecule  has  been  determined; 
and,  now  that  the  result  is  expressed  in  symbols,  noth- 
ing is  easier  than  to  interpret  what  they  have  to  tell 
us.  To  begin  with  the  sodic  carbonate,  Na2CO3.  The 
weight  of  this  molecule  is  2x23  +  12  +  3x16  =  46  + 
12  +  43  =  106  m.c.  The  weight  of  the  molecule  HC1 
is  1  +  35.5  =  36.5,  and  two  such  molecules  would  weigh 
73  m.c.  Next,  for  the  products,  we  have  Nad  =  23 -t- 
35.5  =  58.5,  and  2Nad  =  117.0,  also  CO2  =  12  +  32 
=  44,  and  H2O  =  2  +  16  =  18.  Hence  the  terms  of  our 
equation  stand  for  the  weights  written  over  them  below : 

106  73  117  18  44 

(lsra2CO3  +  2HC1  +  Aq.)  =  (2NaCl  +  HaO  +  Aq.)  +  CO2. 

We  leave  out  of  the  account  the  water  represented 
by  Aq.,  for  this,  being  merely  the  medium  of  the  reac- 
tion, is  not  changed.  Now  we  can  prove  our  work ; 
because,  if  we  have  added  correctly,  the  sum  of  the 
weights  of  the  factors  must  exactly  equal  the  sum  of 
the  weights  of  the  products — and  so  it  is  106  +  73  = 
179,  and  117  +  18+44  =  179.  Besides  the  information 
which  the  equation  gives  us  in  regard  to  the  manner 
in  which  the  chemical  change  takes  place,  the  symbols 
also  inform  us  that  106  parts  by  weight  of  sodic  car- 
bonate are  acted  upon  by  73  parts  by  weight  of  hydro- 


CHEMICAL  ARITHMETIC.  167 

chloric  acid,  and  that  the  yield  is  117  parts  of  common 
salt,  18  parts  of  water,  and  44  parts  of  carbonic-dioxide 
gas. 

We  learn  from  this,  in  the  first  place,  the  exact 
proportion  in  which  the  sodic  carbonate  and  hydro- 
chloric acid  can  be  most  economically  used ;  for,  if  the 
least  excess  of  one  or  the  other  substance  over  the  pro- 
portions indicated  is  taken,  that  excess  will  be  wasted. 
It  will  not  enter  into  the  chemical  change,  but  will  be 
left  behind  with  the  salt  and  water. 

Assume,  then,  that  we  have  500  grammes  of  sodic 
carbonate,  and  we  wish  to  know  what  amount  of  hy- 
drochloric acid  to  use,  we  simply  make  the  proportion 
as  106  :  73  =  500  :  x  =  344^^.  Again,  suppose  wre 
wish  to  know  how  much  common  salt  would  be  pro- 
duced from  these  amounts  of  sodic  carbonate  and  acid, 
we  write  a  similar  proportion — 

106  :  117  =  500  :  x  =  552,  nearly. 

So,  then,  in  any  process,  after  we  have  written  the 
reaction  as  above,  if  the  weight  of  any  factor  or  prod- 
uct is  given,  we  can  calculate  the  weight  of  any  other 
factor  or  product  by  this  simple  rule : 

As  the  total  molecular  weight  of  the  substance  given 
is  to  the  total  molecular  weight  of  the  substance  required, 
so  is  the  given  weight  to  the  required  weight.  By  total 
molecular  weight  we  mean,  evidently,  not  the  weight 
of  a  single  molecule,  but  the  weight  of  the  number  of 
molecules  which  the  equation  indicates. 

This  may  be  called  the  golden  rule  of  chemistry, 

In  the  laboratory  we  never  mix  our  materials  at 
random,  but  always  weigtfi  out  the  exact  proportions 
found  by  this  rule.  When  one  of  the  products  is  a 
gas,  as  in  the  present  case,  a  simple  modification  of  the 


168  CHEMICAL   REACTIONS. 

rule  enables  us  to  calculate  the  volume  of  the  resulting 
gas.  Suppose,  for  example,  we  wished  to  calculate 
what  volume  of  carbonic-dioxide  gas  could  be  obtained 
from  500  grammes  of  sodic  carbonate.  We  should  first 
find  the  weight  by  the  above  rule : 

106  :  44  =  500  :  x  =  207i,  nearly. 

The  answer  is  207^  grammes  of  carbonic  dioxide. 
To  find  the  corresponding  volume  in  litres,  we  have 
merely  to  divide  this  value  by  the  weight  of  one  litre 
of  the  gas.  Now,  there  are  tables,  in  which  the  weight 
of  one  litre  of  each  of  the  common  gases  is  given ;  but 
such  tables,  although  convenient,  are  not  necessary, 
when,  as  in  a  written  reaction,  we  know  the  molecular 
weights  of  the  substances  with  which  we  are  dealing. 
You  remember  that  the  molecular  weight  is  always 
twice  the  specific  gravity  with  reference  to  hydrogen. 
Half  the  molecular  weight  is}  then,  the  specific  gravity 
with  reference  to  hydrogen.  For  example,  the  molecu- 
lar weight  of  carbonic  dioxide  (CO2)  is  44,  and  its  spe- 
cific gravity  with  reference  to  hydrogen  22 — in  other 
words,  a  litre  of  carbonic  dioxide  weighs  22  times  as 
much  as  a  litre  of  hydrogen.  Now,  a  litre  of  hydro- 
gen, under  the  normal  pressure  of  the  atmosphere,  and 
at  the  freezing-point  of  water,  weighs  one  crith,  or 
0.0896  gramme,  or,  near  enough  for  common  purposes, 
0,09  gramme.  If,  then,  a  litre  of  carbonic  dioxide 
is  22  times  as  heavy,  its  weight  is  22  criths,  or  22  x 
0.09  =  1.98  gramme.  Our  total  product,  above,  be- 
ing 207-|  grammes,  the  number  of  litres  will  be  207-J-  -v- 
1.98,  or  verv  nearly  104  litres.  A  litre,  as  I  have  said, 
is  very  nearly  If  pint,  but  we  always  use  these  French 
weights  and  measures  in  the  laboratory,  so  that  the 
values  are  as  significant  to  the  chemist  as  are  pounds 


DECOMPOSITION  OF  CARBONIC  DIOXIDE.  169 

and  pints  to  the  trader.  The  general  rule,  then,  is 
this :  We  first  find  the  weight  of  one  litre  of  the  gas  in 
grammes,  by  simply  multiplying  one-half  of  its  molec- 
ular weight  by  jf^-,  and  then  we  reduce  the  weight  of 
the  gas  in  grammes  to  litres  by  dividing  the  weight  by 
this  product. 

Let  us  pass,  now,  to  another  case  of  chemical 
change,  and  the  example  which  I  have  selected  is 
closely  related  to  the  last.  One  of  the  products  of 
that  reaction  was  carbonic-dioxide  gas,  and  here  we 
have  a  jar  of  that  aeriform  substance.  On  the  other 
hand,  I  have  in  this  bottle  an  elementary  substance, 
called  sodium.  It  belongs  to  the  class  of  metals,  and  is 
one  of  the  constituents  of  sodic  carbonate,  which  we 
used  in  the  former  experiment.  I  now  propose  to 
cause  these  two  substances  to  act  chemically  upon  each 
other;  but,  as  before,  no  chemical  action  will  result 
unless  the  molecules  have  sufficient  freedom  of  motion. 
Those  of  the  carbonic  dioxide  gas  are  already  as  free  as 
the  wind,  moving  with  immense  velocity  through  this 
jar.  But  not  so  with  those  of  the  sodium.  In  the 
usual  solid  condition  of  this  metal,  the  motion  of  its  mol- 
ecules is  restricted  within  very  narrow  limits.  Before, 
we  gave  freedom  to  the  molecules  of  sodic  carbonate  and 
hydrochloric  acid  by  dissolving  the  substances  in  water. 
That  method  is  not  applicable  here,  for  sodium  acts 
chemically  on  water,  and  with  great  violence ;  but  we 
can  reach  a  similar  result  by  melting  the  sodium,  and 
heating  the  molten  metal  until  it  begins  to  volatilize. 
Then,  on  introducing  the  crucible  containing  the  seeth- 
ing metal  into  the  gas,  the  molecules  of  the" sodium,  as 
they  are  forced  up  by  the  heat,  will  come  into  contact 
with  those  of  the  carbonic  dioxide,  and  a  violent  chemi- 
cal action  will  be  the  result. 
13 


170  CHEMICAL   REACTIONS. 

This  action  is  made  evident  to  you  by  the  brilliant 
light  evolved,  and  the  sodium,  as  you  would  say,  is 
burning  in  the  carbonic-dioxide  gas.  Let  us  now  rep- 
resent this  chemical  change  by  our  symbols. 

Beginning  with  the  factors,  the  molecule  of  carbonic 
dioxide,  as  already  stated,  is  represented  by  the  symbol 
CO2.  The  weight  of  the  molecule  of  sodium  has  not 
yet  been  accurately  determined ;  and,  in  the  absence 
of  exact  information,  we  will  assume,  as  is  most  prob- 
able, that  the  molecular  weight  is  twice  the  atomic 
weight,  or,  in  other  words,  that  the  molecules  consist 
of  two  atoms,  Na-Na.  Passing,  next,  to  the  products, 
we  find  only  two,  charcoal,  and  a  substance  called 
sodic  oxide.  As  regards  the  last,  we  have  every  rea- 
son to  believe  that  its  molecules  consist  of  two  atoms 
of  sodium  united  to  a  single  atom  of  oxygen,  Na2O. 
About  the  charcoal  molecules,  we  have  no  means  of  form- 
ing even  a  probable  inference ;  and  we  will,  therefore, 
as  is  usual  in  such  cases,  represent  them  as  consisting  of 
single  atoms.  Hence,  writing  the  products  after  the 
factors,  we  have — 

CO2  ISTa-Na  0  Na2O. 

Carbonic  Dioxide.       Sodium.  Carbon.       Sodic  Oxide, 

Remembering,  now,  that  the  number  of  atoms  on 
the  two  sides  must  be  the  same,  it  is  evident  that  the 
amount  of  oxygen  in  a  molecule  of  CO2  will  yield 
2Na2O ;  and,  further,  that,  to  form  two  molecules  of 
Na2O,  two  molecules  of  Na-Na  are  necessary.  Hence 
our  reaction  must  be  written : 

C02  +  SSTa-Na  =  C  +  2Na2O. 

By  this  we  learn  that,  from  one  molecule  of  carbonic 
dioxide  (CO2)  and  two  molecules  of  sodium  (2Na-Na), 
there  are  formed  two  molecules  of  sodic  oxide  (2Na2O) 


CHEMICAL  RELATIONS  OF  CARBON.  171 

and  one  atom  of  carbon  (C).  It  is  probable  that  the 
atoms  of  carbon  group  themselves  into  molecules  ;  but, 
as  we  know  nothing  about  their  constitution,  we  can- 
not express  it  by  our  symbols. 

Both  of  the  products  of  this  process  are  solids,  and 
will  be  found,  at  the  close  of  the  experiment,  in  the 
small  iron  crucible  in  which  the  sodium  was  melted 
and  introduced  into  the  jar  of  carbonic-dioxide  gas. 
The  sodic  oxide  is  a  white  solid,  which  is  very  soluble 
in  water,  or,  rather,  combines  with  water  to  form  what 
is  called  caustic  soda,  which  dissolves  in  the  liquid;  and 
caustic  soda,  as  you  well  know,  is  a  very  important 
chemical  agent.  But  the  chief  interest  in  this  experi- 
ment centres  about  the  other  product.  Charcoal  is  one 
of  the  forms  of  carbon ;  and  the  peculiar  chemical  re- 
lations of  this  element,  which  are  illustrated  by  our  ex- 
periment, are  not  only  highly  interesting  in  themselves, 
but  have  an  important  bearing  on  the  subject  of  these 
lectures.  I  shall,  therefore,  digress  for  a 'moment  from 
my  immediate  topic,  in  order  to  bring  these  facts  to 
your  notice. 

Carbon,  as  you  probably  know,  is  one  of  the  most 
remarkable  of  the  chemical  elements.  In  the  first 
place,  it  is  most  protean  in  the  outward  aspects  which 
it  assumes.  These  brilliant  crystals  of  diamond,  the 
hardest  of  all  bodies ;  this  black  graphite,  as  extreme 
in  softness  as  is  the  diamond  in  hardness ;  these  still 
more  familiar  lumps  of  coal,  are  all  formed  of  the  same 
elementary  substance.  In  the  second  place,  the  various 
forms  of  fnel  used  on  the  earth  also  consist  chiefly  of 
this  element,  which  is,  therefore,  the  great  source  of  our 
artificial  light  and  heat,  and  the  reservoir  of  that  en- 
ergy w^hich,  by  the  aid  of  the  steam-engine,  man  uses 
with  such  effect. 


172  CHEMICAL   REACTIONS. 

All  carbonaceous  materials  used  as  fuel,  whether 
wood,  coal,  oil,  or  gas,  if  not  themselves  visibly  organ- 
ized, were  derived  from  organized  structures,  chiefly 
plants ;  and  all  the  light,  all  the  heat,  all  the  power, 
which  they  are  capable  of  yielding,  were  stored  away 
during  the  process  of  vegetable  growth.  The  origin 
of  all  this  energy  is  the  sun,  and  it  is  brought  to  the 
earth  by  the  sun's  rays.  Coal  is  the  charred  remains 
of  a  former  vegetation,  and  the  energy  of  our  coal-beds 
was  accumulated  during  long  periods  in  the  early  ages 
of  the  geological  history  of  the  earth.  Wonderful  as 
the  truth  may  appear,  it  is  no  less  certain  that  the 
energy  which  drives  our  locomotives  and  forces  our 
steamships  through  the  waves  came  from  the  sun,  than 
that  the  water,  which  turns  the  wheels  of  the  Lowell 
factories,  came  from  the  springs  of  the  New-Hampshire 
hills.  How  it  comes,  how  there  can  be  so  much  power 
in  the  gentle  influences  of  the  sunbeam,  is  one  of  the 
great  mysteries  of  Nature.  We  believe  that  the  effect 
is  in  some  way  connected  with  the  molecular  structure 
of  matter ;  but  our  theories  are,  as  yet,  unable  to  cope 
with  the  subject.  That  the  power  comes  from  the  sun, 
we  know ;  and,  moreover,  we  are  able  to  put  our  finger 
on  the  exact  spot  where  the  mysterious  action  takes 
place,  and  where  the  energy  is  stored ;  and  that  spot, 
singular  as  it  may  appear,  is  the  delicate  leaf  of  a  plant. 

This  same  carbonic  dioxide,  on  which  we  are  here 
experimenting,  is  the  food  of  the  plant,  and,  indeed, 
the  chief  article  of  its  diet.  The  plant  absorbs  the  gas 
from  the  air,  into  which  it  is  constantly  being  poured 
from  our  chimneys  and  lungs,  and  the  sun's  rays,  act- 
ing upon  the  green  parts  of  the  leaf,  decompose  it. 
The  oxygen  it  contains  is  restored  to  the  atmosphere, 
while  the  carbon  remains  in  the  leaf  to  form  the  struct- 


LATENT   ENERGY   IN   COAL.  173 

ure  of  the  growing  plant.     This  change  may  be  repre- 
sented thus : 

CO2        =        C         +         O=0. 

Carbonic  Dioxide.  Carbon.  Oxygen. 

Now,  to  tear  apart  the  oxygen  atoms  from  the 
carbon  requires  the  expenditure  of  a  great  amount 
of  energy,,  and  that  energy  remains  latent  until  the 
wood  is  burned ;  and  then,  when  the  carbon  atoms 
again  unite  with  oxygen,  the  energy  reappears  undi- 
minished  in  the  heat  and  light,  which  radiate  from  the 
glowing  embers.  Just  as,  when  a  clock  is  wound  up, 
the  energy  which  is  expended  in  raising  the  weight  re- 
appears when  the  weight  falls  ;  so  the  energy,  which  is 
expended  by  the  sun  in  pulling  apart  the  oxygen  and 
carbon  atoms,  reappears  when  those  atoms  again  unite. 
This  is  one  of  the  most  wonderful  and  mysterious  ef- 
fects of  Nature ;  for,  although  the  process  goes  on  so 
silently  and  unobtrusively  as  to  escape  notice,  it  accom- 
plishes an  amount  of  work  compared  with  which  most 
of  the  noisy  and  familiar  demonstrations  of  power  are 
mere  child's-play.  It  is  one  of  the  greatest  achieve- 
ments of  modern  science,  that  it  has  been  able  to  meas- 
ure this  energy  in  the  terms  of  our  common  mechanical 
unit,  the  foot-pound ;  and  we  know  that  the  energy 
exerted  by  the  sun  and  rendered  latent  in  each  pound 
of  carbon,  which  is  laid  away  in  the  growing  wood, 
would  be  adequate  to  raise  a  weight  of  five  thousand 
tons  one  foot. 

The  chief  interest  connected  with  the  experiment 
before  us  is  to  be  found  in  the  fact  that  it  is  almost  the 
parallel  to  the  process  which  is  going  on  in  the  leaf  of 
every  plant  that  waves  in  the  sunshine.  Compare  the 
two  reactions  as  they  are  here  written,  the  one  over 
the  other : 


174  CHEMICAL  REACTIONS. 


CO2  +  2Na-ffa  =  C 
CO2  =  C  +  O=O. 

In  the  first,  the  cause  of  the  breaking  up  of  the  CO2 
molecule  is  evident.  The  molecules  of  the  sodium 
have  what  is  called  an  intense  affinity  for  the  atoms 
of  oxygen,  and  attract  them  with  such  power  as  to  tear 
them  away  from  the  atom  of  carbon.  Now,  when  you 
remember  that  the  atoms  of  carbon  and  oxygen  are 
united  by  such  a  force  that  it  requires  the  great  energy 
I  have  described  to  tear  them  apart,  and  in  the  light 
of  this  knowledge  study  the  second  reaction,  you  will 
fail  to  find  in  the  symbols  any  adequate  explanation  of 
the  effect.  And  they  cannot  explain  it  ;  for  the  sun's 
energy  cannot  be  expressed  by  a  chemical  formula.  But, 
yet,  this  energy  does  here  precisely  the  same  work 
which  the  sodium  accomplishes  in  our  crucible.  More- 
over, there  is  another  striking  analogy  between  the  two 
processes,  which  must  not  be  overlooked. 

The  carbonic  dioxide  is  decomposed  in  a  vegetable 
leaf;  and,  of  the  two  products  of  the  reaction,  the  oxy- 
gen gas  escapes  into  the  air,  while  the  carbon  is  depos- 
ited in  the  vegetable  tissue.  This  relation  between 
the  two  products  depends  on  the  aeriform  condition  of 
oxygen  on  the  one  hand,  and  the  great  fixity  of  carbon 
on  the  other.  Carbon  is  peculiar  in  this  respect  :  In  all 
its  conditions,  whether  of  diamond,  graphite,  or  coal,  it 
is  one  of  the  most  fixed  solids  known.  Even  when  ex- 
posed to  the  highest  artificial  heat,  it  never  loses  its 
solid  condition,  and  so  the  molecules  of  carbon,  as  they 
form  in  the  leaf,  assume  their  native  immobility,  and 
become  a  part  of  the  skeleton  of  the  growing  plant. 
To  fully  appreciate  this  remarkable  relation  of  carbon 
to  organic  structures,  you  must  recall  the  fact  that  the 
only  other  three  elementary  substances,  of  which  ani- 


INFUSIBILITY   OF   CARBON.  175 

mals  and  plants  chiefly  consist — oxygen,  hydrogen,  and 
nitrogen — are  not  only  aeriform,  but  they  are  gases, 
which  no  amount  of  mechanical  pressure  alone  is  able 
to  reduce  to  the  liquid  condition.  All  organized  beings 
may  be  said  to  be  skeletons  of  carbon,  which  have  con- 
densed around  the  carbon  atoms  the  elements  of  water 
and  of  air. 

This  point  is  one  of  such  interest  that  a  familiar 
illustration  of  it  may  be  acceptable.  "When  a  piece  of 
wood  is  heated  out  of  contact  with  the  air,  the  volatile 
elements,  hydrogen,  oxygen,  and  nitrogen,  are  driven 
off  in  various  combinations,  while  the  carbon  molecules 
are  left  behind,  retaining  the  same  relative  position 
they  had  in  the  tree ;  and,  if  we  examine  the  charcoal 
with  a  microscope,  we  shall  find  that  it  has  preserved 
the  forrns  and  markings  of  the  cells,  and  the' rings  of  an- 
nual growth ;  and,  in  fact,  all  those  details  of  structure 
which  marked  the  kind  of  wood  from  which  it  was  made. 

My  assistant  has  projected  on  the  screen  a  magni- 
fied image  of  a  thin  section  of  wood,  which  has  been 
thoroughly  carbonized,  and  you  see  how  strikingly  the 
facts  I  have  stated  appear. 

Now,  just  as  the  non-volatile  carbon  is  deposited 
from  the  carbonic  dioxide  in  the  cell  of  the  plant,  so  in 
our  experiment  is  it  deposited  in  the  crucible.  Both 
of  the  products  of  the  reaction  are  to  a  great  extent 
fixed,  but  the  carbon  by  far  the  most  so ;  and,  in  this 
experiment,  all,  or,  at  least,  a  great  part,  of  the  carbonic 
dioxide,  which  previously  filled  the  jar,  has  deposited 
the  carbon  it  contained  in  the  iron  crucible.  In  the 
plant  the  carbonic  dioxide,  which  passes  through  the 
structure  in  the  process  of  plant-life,  leaves  its  carbon 
in  the  leaf  or  stalk  ;  and  so  here,  the  carbonic  dioxide, 
which  is  brought  by  the  currents  in  the  jar  in  contact 


176  CHEMICAL   REACTIONS. 

with  the  heated  sodium,  leaves  its  carbon  in  the  cruci- 
ble. In  order  to  show  you  that  carbon  has  been  thus 
formed,  I  will  now  remove  the  crucible,  and  quench  it 
with  water.  The  sodic  oxide  (Na2O)  dissolves,  and  the 
charcoal  is  set  free,  and  you  see  that  the  water  in  this 
jar  is  black  with  the  particles  of  floating  charcoal. 

Let  us  now  pass  on  to  study  a  remarkable  series  of 
chemical  changes,  in  which  carbonic  dioxide  also  plays 
an  important  part.  The  first  of  the  series  is  one  with 
which  you  are  all  so  familiar,  that  it  is  perhaps  not  im- 
portant to  repeat  it  in  this  place ;  but,  as  I  am  anxious 
that  you  should  have  the  processes  we  are  studying 
presented  to  you  in  visible  form,  I  will  make  the  trivial 
experiment  of  slaking  some  common  lime. 

The  action  is  very  violent,  and  great  heat  is  devel- 
oped. As  we  shall  hereafter  see,  the  evolution  of  heat 
is  an  indication  of  chemical  combination,  and,  in  the 
case  before  us,  the  lime  unites  with  the  water.  Let  us 
try  to  represent  this  change  by  our  symbols. 

Lime  is  a  compound  of  a  metal  we  call  calcium  and 
oxygen.  It  is,  in  a  word,  a  metallic  ore ;  and  I  have  a 
small  bit  of  the  metal  which  it  contains  in  this  tube. 
By  projecting  an  image  of  the  tube  on  the  screen,  you 
can  see  almost  all  that  I  can,  save  only  that  the  metal 
has  a  brilliant  lustre  and  ruddy  tint,  like  bismuth.  A 
molecule  of  lime  is  formed  of  two  atoms,  one  of  this 
metal  and  the  other  of  oxygen.  Hence  the  symbol 
CaO.  A  molecule  of  water,  as  we  know,  is  represented 
by  H2O.  The  product  of  the  reaction  is  a  light,  white 
powder  we  familiarly  call  slaked  lime,  and  its  analysis, 
interpreted  by  its  chemical  relations,  shows  that  it  has 
the  constitution  CaO2H2.  The  chemical  name  is  calcic 
hydrate,  and  the  change  by  which  it  was  produced  we 
can  now  express  thus : 


PRODUCTION    OF  CHALK.  177 

CaO       +       H2O       =       Ca02H2. 

Lime.  Water.  Calcic  Hydrate. 

In  this  reaction,  as  you  see,  two  molecules  unite  to 
form  a  third,  which  consists  of  the  atoms  of  the  other 
two.  If,  now,  we  mix  this  slaked  lime  with  a  larger 
body  of  water,  the  result  is  an  emulsion  called  milk-of- 
lime,  and  consisting  merely  of  particles  of  the  hydrate 
suspended  in  water.  A  part  of  the  hydrate  actually 
dissolves ;  and,  if  we  employ  as  much  as  700  times  its 
volume  of  water,  the  whole  dissolves,  forming  a  trans- 
parent solution.  This  milk-of-lime,  then,  is  a  solu- 
tion of  calcic  hydrate,  containing  a  large  excess  of  the 
solid  hydrate  in  suspension.  But  there  is  a  very  sim- 
ple means  of  separating  the  solid  from  the  solution. 

We  use  for  the  purpose  a  circular  disk  of  porous 
paper,  called  a  filter,  which  we  fold  in  the  shape  of  a 
cone,  and  place  in  a  glass  funnel.  On  pouring  the  tur- 
bid liquid  into  the  paper  cone,  the  clear  solution  will 
trickle  through  the  pores  of  the  paper,  but  the  solid 
sediment  will  be  retained  on  the  upper  surface. 

Having  now  obtained  a  clear  solution  of  calcic  hy- 
drate (CaO2H2  +  Aq),  I  propose  to  show  you  next  the 
action  of  carbonic  dioxide  upon  it.  I  have  already  shown 
you  this  reaction  as  a  test  for  carbonic  dioxide,  but  we 
will  now  study  the  chemical  process  more  in  detail. 
Pouring  our  clear  solution  into  this  jar,  we  will  pour  in 
after  it  a  quantity  of  carbonic  dioxide,  which,  although 
a  gas,  is  so  heavy  that  we  can  handle  it,  as  you  remem- 
ber, very  much  like  a  liquid.  The  gas  is  now  resting  on 
the  solution,  but  the  action  is  exceedingly  slow ;  for, 
although  the  particles  of  the  calcic  hydrate  are  free  to 
move  in  the  liquid,  and  those  of  the  carbonic  dioxide 
in  the  space  above  the  liquid,  yet  each  is  restricted  to 
those  spaces,  and  the  two  sets  of  molecules  cannot 
come  in  contact,  except  at  the  surface  of  separation. 


178  CHEMICAL  REACTIONS. 

But,  let  us  shake  up  the  liquid,  so  as  to  bring  the  mole- 
cules of  both  liquid  and  gas  in  contact,  and  you  see 
that,  at  once,  we  have  a  very  marked  change.  The 
liquid  becomes  turbid,  and,  after  a  while,  a  quantity  of  a 
white  powder  will  fall  to  the  bottom,  which,  if  collected 
and  examined,  will  be  found  to  be  identical  with  chalk. 
Now  that  you  are  acquainted  with  our  method  of  no- 
tation, I  can  best  explain  to  you  this  change  by  writing 
at  once  the  reaction : 

(OaOaHa  +  C02  +  Aq.)  =  OaCOs  +  (H20  +  Aq.). 

Calcic  Hydrate.  Calcic  Carbonate. 

The  symbols  of  the  factors  of  the  reaction  you  will 
at  once  recognize,  and  you  will  also  interpret  the 
meaning  of  Aq.,  used  to  indicate  that  the  calcic  hy- 
drate and  carbonic  dioxide  come  together  in  solution. 
Among  the  products  of  the  reaction,  the  first  symbol 
represents  one  molecule  of  calcic  carbonate,  the  mate- 
rial of  chalk.  This  body,  being  insoluble  in  water, 
drops  out  of  the  solution,  and  forms  what  is  called  a 
precipitate,  a  condition  which  we  indicate  arbitrarily 
by  drawing  a  line  under  the  symbol.  The  only  other 
product  of  the  reaction  is  water,  which,  of  course,  min- 
gles with  the  great  mass  of  water  present,  and  this  we 
express  by  H2O  +  Aq. 

I  need  not  tell  you  that  this  white  powder  is  not 
only  the  material  of  chalk,  but  the  material  of  the 
limestone-rocks,  which  form  so  great  a  part  of  the 
rocky  crust  of  our  globe.  Not  only  the  rough  moun- 
tain limestones,  but  the  fine  marbles,  and  that  beauti- 
ful, transparent,  crystalline  mineral  we  call  Iceland- 
spar,  are  aggregates  of  molecules,  having  the  same  con- 
stitution as  those  which  have  formed  in  this  experi- 
ment. The  differences  of  texture  may,  doubtless,  be 
referred  to  differences  of  molecular  aggregation  ;  but 


CHALK   DISSOLVES   IN   SODA-WATER.  179 

we  have  not  yet  been  able  to  discover,  either  what  the 
difference  is,  or  on  what  it  depends. 

In  order  to  produce  the  last  reaction,  we  poured  the 
gas  upon  the  solution  of  calcic  hydrate ;  and  the  chalk 
was  only  produced  as  fast  as  the  gas  dissolved  in  the 
liquid.  We  shall  obtain  the  reaction  more  promptly, 
if,  instead  of  taking  the  gas  itself,  we  employ  a  solution 
of  the  gas  in  water,  previously  prepared.  Moreover, 
this  form  of  the  experiment  will  enable  me  to  show 
you  a  phase  of  the  process  which  might  otherwise  es- 
cape your  notice.  I  need  not  tell  you  that  we  can 
easily  obtain  such  a  solution  ready-made  to  our  hands. 
That  beverage,  which  we  persist  in  miscalling  soda- 
water,  is  simply  an  over-saturated  solution  of  carbonic 
dioxide  in  water,  made  by  forcing  a  large  excess  of  the 
gas  into  a  strong  vessel  filled  with  water.  At  the  or- 
dinary pressure  of  the  air,  water  will  dissolve  its  own 
volume  of  this  gas ;  but,  when  forced  in  by  pressure, 
the  water  dissolves  an  additional  volume  for  every 
additional  atmosphere  of  pressure.  As  soon,  how- 
ever, as  this  solution  is  drawn  out  into  the  air,  the  ex- 
cess of  gas  above  one  volume  escapes,  causing  the  effer- 
vescence with  which  we  are  so  familiar.  Carbonic  di- 
oxide is  formed  in  the  process  of  fermentation  by 
which  beer  and  wine  are  prepared ;  and  it  is  the  es- 
cape of  the  excess  of  this  gas,  dissolved  under  pressure, 
which  causes  the  effervescence  of  bottled  beer  and 
champagne.  The  solution  in  water  (soda-water)  is  now 
supplied  to  the  market  in  bottles  called  siphons,  which 
are  convenient  for  our  purpose. 

Notice  that,  as  I  permit  the  solution  to  flow  into 
the  lime-water,  the  same  white  powder  appears  as  be- 
fore ;  but,  now,  notice  further  that,  as  I  continue  to 
add  the  solution  of  carbonic  dioxide,  this  white  solid 


180  CHEMICAL  REACTIONS. 

redissolves,  and  we  have  a  beautifully  clear  solution. 
It  is  generally  believed  that,  under  these  conditions, 
in  presence  of  a  great  excess  of  carbonic  dioxide,  the 
molecule  of  calcic  carbonate  combines  with  additional 
atoms  of  carbon,  oxygen,  and  hydrogen,  to  form  the 
very  complex  molecule  H2CaC2O6,  which  is  assumed  to 
be  soluble  in  water ;  but,  as  this  point  is  one  of  doubt, 
I  prefer  to  present  the  phenomenon  to  you  as  simply 
one  of  solution,  and  as  illustrating  a  remarkable  point 
in  our  chemical  philosophy — the  fact  that  the  produc- 
tion of  a  given  compound  is  frequently  determined  by 
the  circumstance  of  its  insolubility.  The  calcic  carbon- 
ate forms,  in  the  first  instance,  because  this  compound  is 
insoluble ;  but,  when  a  proper  solvent  like  the  aerated 
water  is  present  in  sufficient  excess,  no  such  compound 
results,  or,  at  least,  we  have  no  evidence  of  its  forma- 
tion. 

Most  of  my  audience  will  be  more  interested,  how- 
ever, in  this  solution  of  chalk  in  soda-water  (for  such  it 
is),  from  the  fact  that  it  plays  a  very  important  part  in 
Nature,  and  is  a  common  feature  of  domestic  experi- 
ence. Such  a  solution  as  this  is  what  we  call  hard 
water,  and  spring-water  is  frequently  in  this  condition. 
Such  water  is  said  to  kill  soap,  and  is  disagreeable  when 
used  in  washing,  because  the  lime  in  solution  forms  with 
the  fatty  constituent  of  the  soap  an  insoluble,  sticky 
mass,  which  adheres  to  the  hands  or  cloth.  Moreover, 
when  such  water  is  boiled,  the  carbonic  dioxide  is  driven 
off,  and  the  water  loses  its  power  of  holding  the  chalk 
in  solution,  which  is  deposited  sometimes  as  a  loose 
powder,  but  at  other  times  as  a  hard  crust  on  the 
sides  of  the  boiler. 

I  cannot  readily  show  you  the  reprecipitation  un- 
der these  conditions ;  but  I  have  here  a  crust,  which 


HOW  LIMESTONES  MAY  BE  FORMED.  181 

was  formed  in  a  steam-boiler  in  the  manner  I  have  de- 
scribed. A  precisely  similar  action  gives  rise  to  the 
formation  of  stalactites  in  lime-caverns,  and  of  a  form 
of  lime-rock  called  travertine.  Some  of  the  finest  mar- 
bles have  been  formed  in  this  way. 

Thus  it  is  that  we  have  been  imitating  here  the 
production  of  chalk,  limestone,  and  marble,  at  least 
so  far  as  the  chemical  process  is  concerned.  The  mole- 
cule of  all  these  substances  has  the  same  constitution, 
expressed  by  the  symbol  CaCO3.  Now,  it  is  evident 

that 

CaC03        =        CaO        +        CO.. 

Calcic  Carbonate.  Lime.  Carbonic  Dioxide. 

I  mean  simply  by  this,  that  it  is  theoretically  possi- 
ble to  form,  from  one  molecule  of  calcic  carbonate,  one 
molecule  of  lime  and  one  molecule  of  carbonic  dioxide ; 
but  it  does  not  follow  from  this  that  it  is  practically  pos- 
sible to  break  up  the  molecule  of  calcic  carbonate  in  this 
way ;  and  we  must  avoid  the  error,  not  unfrequently 
made  by  chemical  students,  of  being  led  astray  by  our 
notation.  These  equations,  which  we  call  reactions,  are 
not  like  the  equations  of  algebra.  Any  thing  that  can  be 
deduced  from  an  algebraic  equation,  according  to  the 
rules  of  the  science,  must  be  true ;  but  it  by  no  means 
follows  that  any  combinations  we  may  form  with  our 
symbols  can  be  realized.  We  cannot  deduce  facts  from 
chemical  symbols.  They  are  merely  the  language  by 
which  we  express  the  results  of  experiment;  and  for 
this  reason  I  have  been,  and  shall  be,  very  careful  to 
show  you  the  facts  before  I  attempt  to  express  them  in 
chemical  language.  But,  in  the  case  before  us,  our 
caution  is  needless,  for  we  can  break  up  the  molecule 
in  the  precise  way  which  our  assumed  reaction  indi- 
cates ;  and  I  will  show  you,  lastly,  two  additional 


182  CHEMICAL  REACTIONS. 

chemical  processes,  which  will  bring  back  our  material 
to  the  condition  of  lime  and  carbonic  dioxide,  the  sub- 
stances from  which  wTe  started. 

The  first  is  a  reaction,  identical  with  the  one  I  have 
just  written.  Since  the  beginning  of  the  lecture,  I 
have  been  strongly  heating  some  lumps  of  chalk  in 
this  platinum  crucible.  The  process  is  a  slow  one ;  and 
it  was  necessary  to  begin  the  experiment  early,  in 
order  that  I  might  show  you  the  result.  The  chemical 
change  is  identical,  however,  with  that  which  may  be 
observed  in  any  lime-kiln,  where  lime  is  made  by  burn- 
ing limestone.  Each  molecule  of  chalk,  CaCO3,  looses 
a  molecule  of  carbonic  dioxide,  CO2,  and  we  have  left 
a  molecule  of  lime,  CaO.  But  the  change  in  the  ap- 
pearance of  the  white  mass  produced  by  burning  is  so 
slight  that  I  must  bring  in  the  aid  of  experiment  to 
prove  that  any  change  has  taken  place ;  and,  first  of  all, 
I  must  show  you  the  test  I  am  going  to  use. 

In  the  first  of  these  two  jars  I  have  an  emulsion 
of  chalk,  and  in  the  second  milk-of-lime.  Notice  that 
this  piece  of  paper,  colored  by  a  vegetable  dye  called 
turmeric,  remains  unchanged  when  dipped  in  the  emul- 
sion of  chalk,  but  turns  red  in  the  milk-of-lime. 

Let  us  test,  now,  the  contents  of  our  crucible.  We 
will  first  empty  it  into  some  water.  The  white  lumps 
almost  instantly  become  slaked,  and  render  the  water 
milky.  We  will  now  dip  in  a  sheet  of  turmeric-paper, 
and  you  see  that,  although  we  began  with  inactive 
chalk,  we  have  obtained  a  material  which  acts  on  the 
turmeric-paper  like  caustic  lime.  Thus,  then,  we  have 
regenerated  the  lime. 

Let  us  next  see  if  we  can  regenerate  the  carbonic 
dioxide : 

In  the  last  experiment,  carbonic  dioxide  was  pro- 


DECOMPOSITION   OF   CHALK. 


183 


duced,  but  it  escaped  so  slowly,  and  in  such  small  quan- 
tities, as  entirely  to  escape  notice.  Where,  however, 
limestone  is  burned  on  a  large  scale,  the  current  of  gas 
from  the  kiln  is  frequently  very  perceptible  ;  and  more 
than  one  poor  vagrant,  who  has  sought  a  night's  lodg- 
ing under  the  shelter  of  the  stack,  has  been  suffocated 
by  the  stream.  But  we  can  make  evident  the  produc- 
tion of  carbonic  dioxide  from  chalk  without  the  aid  of 
such  a  sad  illustration. 


FIG.  22.-— Pneumatic  Trough,  with  Two-necked  Gas-bottle. 

In  this  bottle  we  have  some  bits  of  chalk.  One  of 
the  two  necks  of  the  bottle  is  closed  by  a  cork,  through 
which  passes  tightly  an  exit-tube,  to  conduct  away  any 
gas  that  may  be  formed.  The  other  is  also  corked,  and 
through  the  cork  passes  a  funnel-tube,  by  which  I  can 
introduce  any  liquid  reagent  into  the  bottle  (Fig.  22). 
On  pouring  in  some  muriatic  acid,  a  violent  efferves- 
cence ensues,  and  a  gas  is  formed  which,  flowing  from 
the  exit-tube,  displaces  the  water  in  this  glass  bell. 

The  bell  stands  in  what  we  call  a  pneumatic  trough, 
and  this  simple  apparatus  for  collecting  gases  must,  I 
think,  be  familiar  to  all  of  my  audience.  The  open 


184  CHEMICAL   REACTIONS. 

mouth  of  the  bell  rests  on  the  shelf  of  the  trough  un- 
der water,  and  the  liquid  is  sustained  in  it  by  the  press- 
ure of  the  air.  Let  me,  while  the  experiment  is  going 
on,  write  out  the  reaction : 

CaOO3  +  (2HC1  +  Aq.)  =  (CaCl2  +  Aq.)  +  CO^ 

Chalk.          Hydrochloric  Acid.         Calcic  Chloride. 

We  already  know  the  symbols  of  all  the  factors, 
and  we  may,  therefore,  confine  our  attention  to  the 
products. 

The  products  are,  first,  carbonic-dioxide  gas ;  and, 
secondly,  a  solution  in  water  of  a  compound  whose 
molecule  consists  of  calcium  and  chlorine,  and  which 
we  call  calcic  chloride.  And,  now  that  the  jar  is 
filled,  I  can  easily  show  that  we  have  regenerated  car- 
bonic dioxide.  Removing  the  jar  from  the  trough,  we 
will  first  lower  into  it  this  lighted  candle,  and  then 
pour  into  it  some  lime-water.  The  candle  is  instantly 
extinguished,  and  the  lime-water  rendered  turbid. 

Thus  we  end  the  torture  of  these  molecules.  You 
have  seen  how  easily  we  have  formed  them,  and  how 
readily  we  have  broken  them  up.  We  began  with 
lime  and  carbonic  dioxide,  which  we  united  to  form 
chalk.  We  dissolved  the  chalk  in  a  solution  of  CO2, 
and  learned  how,  in  Nature,  various  forms  of  limestone 
could  be  crystallized  from  this  solution.  Lastly,  we 
have  recovered  from  the  chalk  the  lime  and  carbonic 
dioxide  with  which  we  begun.  I  hope  you  have  been 
able  to  follow  these  changes,  and  to  understand  the 
language  in  which  they  are  expressed.  If  so,  we  have 
taken  another  step  in  advance,  and,  at  the  next  lecture, 
shall  be  able  to  go  on  and  classify  these  reactions,  and 
thus  prepare  the  way  by  which  we  may  reach  still  fur- 
ther truth  in  regard  to  this  wonderful  microcosm  of 
molecules  and  atoms. 


NOMENCLATURE   OF  CHEMISTRY.  185 

Before,  however,  closing  my  lecture,  I  will  embrace 
the  opportunity  offered  by  this  division  of  my  subject 
to  explain,  as  briefly  as  I  can,  the  principles  of  our 
chemical  nomenclature.  This  nomenclature  originated 
in  1787  with  a  committee  of  the  French  Academy  of 
Sciences,  a  committee  of  which  the  great  chemist  La- 
voisier was  the  ruling  spirit.  It  was  an  attempt  to  in- 
dicate the  composition  of  a  substance  by  its  name,  and, 
for  half  a  century  after  its  adoption,  it  served  most 
admirably  the  purpose  for  which  it  was  devised,  and 
exerted  a  marked  influence  on  the  development  of 
chemistry.  The  nomenclature  was  based,  however,  on 
the  dualistic  theory,  of  which  Lavoisier  was  the  father, 
and,  when  at  last  our  science  outgrew  this  theory,  the 
old  names  lost  much  of  their  significance  and  appropri- 
ateness. Within  the  last  few  years  attempts  have  been 
made  to  modify  the  old  nomenclature,  so  as  to  better 
adapt  the  names  to  our  modern  ideas.  Unfortunately, 
the  result,  like  most  attempts  to  piece  out  an  old  gar- 
ment, is  far  from  satisfactory,  and  reviewers  revel  in 
the  absurdities  to  which  the  nomenclature  leads  when 
applied  to  many  of  the  products  of  modern  chemical 
investigation.  Fortunately,  however,  chemical  symbols 
now  supply  to  a  great  extent  the  place  of  philosophical 
names,  and  hence  the  nomenclature  is  a  far  less  im- 
portant feature  in  the  new  chemistry  than  it  was  in  the 
old.  I  shall  not,  therefore,  enter  into  much  detail  in 
regard  to  it,  but  limit  myself  to  the  statement  of  a  few 
rules  which  will  give  you  the  key  to  the  significance  of 
the  more  common  chemical  terms. 

The  names  of  elementary  substances  are  necessarily 

arbitrary.    Those  which  were  known  before  1787  retain 

their  old  names,  such  as  sulphur,  phosphorus,  iron,  gold, 

and  several  others,  including  all  the  useful  metals.    Most 

14 


186  CHEMICAL  REACTIONS. 

of  the  more  recently-discovered  elements  have  been 
named  in  allusion  to  some  prominent  property,  or  some 
circumstance  connected  with  their  history:  as  oxygen, 
from  o£t>9  yevvdo)  (acid  generator) ;  hydrogen,  from  vScop 
yevvdco  (water  generator) ;  chlorine,  from  xXwpos  (green) ; 
iodine,  from  IwBrjs  (violet) ;  bromine,  from  /3pw/w  (fetid 
odor).  The  names  of  the  newly-discovered  metals  have  a 
common  termination,  um,  as  potassium,  sodium,  plati- 
num ;  and,  the  names  of  several  of  the  non-metallic  ele- 
ments end  in  ine,  as  chlorine,  bromine,  iodine,  fluorine. 
Passing  next  to  binary  compounds — that  is,  com- 
pounds of  only  two  elements — we  notice,  first,  that  the 
simple  compounds  of  the  other  elements  with  oxygen 
are  all  called  oxides,  and  that,  in  order  to  distinguish 
the  different  oxides,  we  use  adjectives  formed  from  the 
name  of  the  element  with  which  the  oxygen  is  com- 
bined, preferring  however,  in  many  cases,  the  Latin 
name  to  the  English,  both  for  the  sake  of  euphony  and 
in  order  to  secure  more  general  agreement  in  different 
languages.  Thus  we  have — 

Argentic  oxide AgaO 

Plumbic  oxide PbO 

Stannic  oxide Sn02 

When  the  same  element  forms  with  oxygen  two 
compounds  the  termination  ic  is  retained  for  the  higher 
oxide,  while  the  termination  ous  is  given  to  the  lower. 
Thus— 

Ferrous  oxide FeO 

Ferric  oxide FeaOa 

Sulphurous  oxide SOa 

Sulphuric  oxide SO3 

If  there  are  more  than  two  oxides,  or  if,  in  any  case, 
there  are  objections  to  the  use  of  the  termination  ous, 
the  necessary  distinctions  are  made  by  means  of  Greek 
numeral  prefixes  : 


NOMENCLATURE   OF   CHEMISTRY.  187 

"Nitrous  oxide N2O 

Nitric  oxide NO 

Dinitric  trioxide N20s 

Nitric  dioxide NO2 

Dimtric  pentoxide N2Oe 

.Carbomc  oxide CO 

Carbonic  dioxide COa 

The  names  of  the  binary  compounds  of  the  other 
elements  are  formed  like  those  of  the  oxides. 

Compounds  of  Chlorine  are  called  Chlorides. 
Bromine  "  Bromides. 
Iodine  "  lo&ides. 


Fluorine 

Sulphur 

Nitrogen 

Phosphorus 

Arsenic 

Aatimony 

Carbon 


Fluorides. 


Phosphides. 
Arsenides. 
Antimonies. 
Carbonito. 


Moreover,  the  specific  names  in  the  several  classes  of 
compounds  also  follow  the  analogy  of  the  oxides,  thus  : 


chloride  ..............  .  ..........  SnCl2 

Stannic  chloride  ..........................  SnCU 

Diferrous  sulphide  ........................  Fe2S 

Ferrous  sulphide  .........................  FeS 

Ferric  sulphide  ..........................  Fe2S3 

Ferric  disulphide  ........................  FeSa 

And  here,  before  we  pass  on  to  the  names  of  compounds 
of  a  higher  order,  let  me  ask  you  to  carefully  fix  in  your 
memory  the  fact  that  the  termination  ide  always  indi- 
cates a  compound  containing  only  two  elements. 

Of  compounds  of  three  or  more  elements  the  most 
prominent  class  is  that  of  the  acids,  bodies  originally  so 
called  on  account  of  their  sharp  or  acrid  taste.  Now, 
the  greater  part  of  the  inorganic  or  mineral  acids  are 


188  CHEMICAL  REACTIONS. 

composed  of  the  two  elements  hydrogen  and  oxygen, 
united  to  some  third  element,  which  is  the  characteristic 
constituent  in  each  case ;  and,  from  this  third  element 
the  acid  takes  its  name,  the  terminations  ic  and  ous 
being  used  as  in  the  case  of  binaries  to  indicate  a  greater 
or  less  amount  of  oxygen  in  the  compound.  Thus  we 
have — 

Nitrous  acid HNO2 

Nitric  acid HNO3 

Sulphurous  acid H2SO3 

Sulphuric  acid H2SO4 

Phosphorous  acid H3PO3 

Phosphoric  acid H3PO4 

In  every  acid  we  can  by  various  chemical  processes 
replace  the  hydrogen  it  contains  with  different  metallic 
elements,  and  we  thus  obtain  a  very  large  class  of  com- 
pounds called  salts.  The  generic  name  of  the  salts  of 
each  acid  is  formed  by  changing  the  termination  ic,  of 
the  name  of  the  acid,  into  ate,  or  the  termination  ous 
into  ite,  thus : 

Sulphurous  acid  forms Sulphites, 

Sulphuric  acid        "     Sulphates, 

Phosphorous  acid  "     Phosphites, 

Phosphoric  acid      "     Phosphates, 

Carbonic  acid         "     Carbonates, 

Silicic  acid  "     Silicates, 

and  the  different  salts  of  the  same  acid  are  distinguished 
by  adjectives  as  before.  For  example  : 

Nitric  acid 

Sodic  nitrate 

Potassic  nitrate KNO3 

Argentic  nitrate AgNOs 

So  also : 

Sulphuric  acid H2S04 

Potassic  sulphate K2SO4 


NOMENCLATURE   OF   CHEMISTRY.  189 

Calcic  sulphate CaSO4 

Mercurows  sulphate Hg2SO4 

Mercuric  sulphate HgSO4 

Yeri'ous  sulphate FeSO4 

Ferric  sulphate Fe2(SO4)3 

The  terminations  ous  and  ic,  used  in  the  names  of 
these  salts,  indicate  the  same  difference  in  the  condition 
of  the  metallic  element  which  determines  the  union  of 
the  metal  with  more  or  less  oxygen.  Ferrous  and  ferric 
sulphates,  for  example,  correspond  to  ferrous  and  ferric 
oxides.  The  nature  of  this  difference  will  be  discussed 
in  the  chapter  on  quantivalence. 

There  is  an  important  class  of  compounds  which 
bears  to  water  a  relation  similar  to  that  which  salts  sus- 
tain to  their  respective  acids.  This  class  of  compounds 
is  called  the  hydrates,  and  may  be  regarded  as  derived 
from  water,  by  replacing  one-half  of  its  hydrogen. 
Thus  we  have — 

Potassic  hydrate KOH      from    HOH 

Calcic  hydrate CaO2H2     "      2HOH 

Bismuthic  hydrate BiO3H3     "      3HOH 

Silicic  hydrate SiO4H4     "      4HOH 

So  also : 

Ferrous  hydrate FeO2H2 

Ferric  hydrate FeaOeHa 

The  very  interesting  theoretical  relations  of  the  hydrates 
will  hereafter  be  discussed. 

When  the  hydrogen  of  an  acid  is  only  in  part  re- 
placed, or  is  replaced  by  more  than  one  metallic  ele- 
ment, the  constitution  of  the  resulting  salt  may  still  be 
indicated  by  the  name,  as  in  the  following  examples  : 

Hydro-disodic  phosphate H,Na2PO4 

Potassio-aluminic  sulphate KaAl2(S04)4 


190  CHEMICAL  REACTIONS. 

In  like  manner  the  relative  proportions  of  the  several 
ingredients  of  a  salt  may  be  indicated,  as  in — 

Tetrahydro-calcic  diphospbate H4Ca(P04)2 

Disodic  tetraborate  (borax) Na2B4O7 

But,  as  is  evident,  names  like  the  last  two  are  prac- 
tically useless,  and,  when  we  attempt  to  extend  the 
nomenclature  to  organic  compounds,  we  are  led  into 
still  greater  absurdities ;  so  that,  although  by  giving 
arbitrary  names  to  various  groups  of  atoms  called  com- 
pound radicals  we  have  been  able,  to  a  limited  extent, 
to  adapt  the  nomenclature  to  this  class  of  substances, 
yet  we  have  been  compelled  in  many  cases  to  resort  to 
trivial  names  like  those  used  before  the  adoption  of  the 
nomenclature.  The  names  oil  of  vitriol,  corrosive  sub- 
limate, calomel,  saltpetre,  borax,  cream-of-tartar,  etc.,  of 
the  last  century,  have  their  counterparts  in  aldehyde, 
glycol,  phenol,  urea,  morphine,  naphthaline,  and  many 
other  familiar  names  of  our  modern  science.  Of  course, 
such  names  are  subject  to  no  rules,  and,  although  they 
have  been  usually  selected  with  care,  and  indicate  by 
their  etymology  important  relations  or  qualities,  they 
must  be  associated  separately  with  the  substances  they 
designate. 


LECTUEE  IX. 

CHEMICAL   CHANGES   CLASSIFIED. 

AMONG  chemical  processes  we  have  already  distin- 
guished two  classes:  1.  Analysis  comprising  those  re- 
actions of  which  the  chief  feature — although  not  neces- 
sarily the  only  feature — is  the  resolution  of  a  compound 
body  into  elementary  substances,  or  else  into  simpler 
compounds ;  2.  Synthesis  including  such  reactions  as 
consist  chiefly  in  the  union  of  elementary  substances  to 
form  compounds,  or  of  simpler  compounds  to  form 
those  which  are  more  complex.  In  addition  to  these 
two,  we  also  distinguish  a  third  class  of  reactions  called 
metathesis,  which  are  chiefly  marked  by  the  substitu- 
tion in  a  compound  of  one  element  for  another  without 
otherwise  disturbing  the  composition  of  the  body,  as 
when,  for  example,  by  substituting  zinc  for  lead,  we 
change  acetate  of  lead  into  acetate  of  zinc,  or  when, 
from  chloride  of  barium  and  sulphate  of  copper,  we 
obtain  chloride  of  copper  and  sulphate  of  barium.  The 
words  analysis,  synthesis,  and  metathesis,  are  derived 
from  the  Greek,  and  signify  respectively  to  tear  apart, 
to  bind  together,  and  to  interchange,  and  with  the  same 
meaning  we  also  speak  of  analytical,  synthetical,  and 
metathetical  reactions. 

This  classification,  however,  is  not  exhaustive,  nor 


192  CHEMICAL   CHANGES   CLASSIFIED. 

are  its  categories  strictly  exclusive  ;  for,  not  only  are 
there  many  reactions  which  cannot  be  included  under 
either  of  the  three  types,  but,  moreover,  chemical  pro- 
cesses are  seldom  limited  to  a  single  mode  of  reaction. 
The  analysis  of  one  compound  is  usually  accompanied 
by  the  synthesis  of  another,  and,  although  examples  of 
simple  metathesis  are  very  common,  yet  the  interchange 
of  elements  implied  by  the  term  is  often  followed  by 
the  breaking  up  of  one  or  both  of  the  resulting  prod- 
ucts. Furthermore,  as  interpreted  by  the  atomic  the- 
ory, every  chemical  change  may  be  regarded  as  the 
breaking  up  of  molecules  into  atoms,  and  the  regroup- 
ing of  these  atoms  to  form  new  molecules,  and  we  re- 
quire no  other  aid  in  representing  any  process  than  that 
which  chemical  symbols  afford.  Nevertheless,  the  terms 
analysis,  synthesis,  and  metathesis,  are  in  such  general 
use  that  it  is  important  to  understand  their  meaning, 
and  the  old  classification  will  be  a  useful  guide  in  lay- 
ing out  our  course  of  study  and  preparing  the  way  for 
a  wider  generalization,  which  will  include  all  subordi- 
nate distinctions.  We  will  study  first  a  few  processes 
of  which  analysis  is  the  predominant  feature,  and  after- 
ward pass  to  others  which  are  equally  characteristic  as 
examples  of  synthesis,  selecting  always  such  examples 
as  incidentally  illustrate  important  principles  or  inter- 
esting facts  of  the  science.  Lastly,  we  will  study  the 
metathetical  reactions,  which  are  not  only  very  common, 
but  frequently  occur  undisturbed  by  other  modes  of 
chemical  change,  and  the  study  of  this  very  important 
class  of  phenomena  will  show  us  some  of  the  latest 
phases  which  our  chemical  philosophy  has  assumed. 

Indeed,  the  great  advances  of  modern  chemistry 
have  been  largely  due  to  the  intelligent  study  of  meta- 
thetical reactions.  As  the  term  metathesis  implies,  these 


PREPARATION   OF  OXYGEN   GAS.  193 

reactions  are  not  caused  by  an  entire  breaking  up  of  the 
molecules  into  atoms,  and  the  production  from  the  wreck 
of  new  molecules  with  a  wholly  different  structure,  but 
are  due  simply  to  an  interchange  between  molecules  of 
certain  of  their  parts,  the  structure  of  the  associated 
molecules  not  being  otherwise  altered.  Hence  it  is  fre- 
quently possible  to  infer  from  the  known  structure  of 
the  factors  of  such  reactions  what  must  be  the  structure 
of  the  products,  or  the  reverse ;  and  by  starting  from 
the  simplest  molecules,  in  regard  to  whose  structure 
there  can  be  no  doubt,  and  following  out  this  principle 
through  a  series  of  reactions,  we  have  been  able  to  reach 
definite  conclusions  in  regard  to  the  structure  of  highly 
complex  products. 

Of  the  analytical  reactions  I  will  select  for  our  first 
illustration  the  process  by  which  oxygen  gas  is  usually 
made.  The  common  source  of  oxygen  is  a  white  salt 
called  potassic  chlorate.  This  salt  has  a  very  soothing 
effect  on  an  irritated  throat,  and  is  perhaps  best  known 
from  the  troches  in  which  it  is  the  active  ingredient, 
but  the  great  mass  of  the  potassic  chlorate  manufactured 
is  used  for  fireworks  or  for  making  oxygen  gas,  and  it 
is  to  the  last  use  we  now  propose  to  apply  it.  For  this 
purpose,  we  have  only  to  heat  the  salt  to  a  low,  red 
heat  in  an  appropriate  vessel.  We  use  here  a  copper 
flask,  and  connect  the  exit-tube  with  the  now  familiar 
pneumatic  trough.  While  my  assistant  is  preparing 
the  oxygen  gas,  I  wrill  explain  to  you  the  process. 

Although  potassic  chlorate  is  a  non-volatile  solid, 
and  we  have  no  direct  means  of  weighing  its  molecules, 
yet,  from  the  purely  chemical  evidence  we  possess, 
there  is  no  doubt  whatever  about  its  molecular  consti- 
tution. It  is  expressed  by  the  symbol  KC1O3,  and, 
in  the  process  before  us,  the  potassic  chlorate  simply 


194  CHEMICAL   CHANGES   CLASSIFIED. 

breaks  up  into  another  salt  called  potassic  chloride  and 
oxygen  gas, 

KC103        =        KC1        +        0», 

Potassic  Chlorate.         Potassic  Chloride.      Oxygen  Atoms. 

that  is,  each  molecule  of  the  salt  gives  a  molecule  of 
potassic  chloride  and  three  atoms  of  oxygen.  Notice 
that  I  say  three  atoms ;  for  this  is  a  point  to  which  I 
must  call  your  attention. 

We  are  not  dealing  here  with  an  example  of  pure 
analysis,  although  that  feature  of  the  reaction  pre- 
dominates over  every  other.  Oxygen  gas  is  the 
product  formed;  and,  as  I  have  several  times  said, 
we  know  that  the  molecules  of  oxygen  consist  of 
two  atoms.  Hence,  the  three  atoms  which  the  heat 
drives  off  must  pair,  and,  from  three  atoms,  we  can 
only  make  one  molecule.  What,  then,  is  to  become  of 
the  third  atom,  which  seems  to  be  left  out  in  the  cold  ? 
You  must  have  already  answered  this  question ;  for 
you  remember  that  our  symbols  only  express  the 
change  in  one  of  the  many  millions  of  molecules 
which  are  breaking  up  at  the  same  instant ;  so  there 
can  be  no  want  of  a  mate  for  our  solitary  atom.  In- 
deed, two  molecules  of  chlorate  will  give  us  just  the 
number  of  atoms  we  want  to  make  three  molecules  of 
oxygen  gas.  Hence,  we  should  express  the  change 
more  accurately  by  doubling  the  symbols : 

2KC1O3         =         2KC1         +         BOO. 

Potassic  Chlorate.  Potassic  Chloride.  Oxygen  Gas. 

Let  me  next  remind  you  that  these  symbols  express 
exact  quantitative  relations ;  and,  as  some  of  my  young 
friends  mny  desire  to  know  how  to  calculate  the  amount 
of  chlorate  they  ought  to  use  in  order  to  make  a  given 
volume,  say,  ten  litres  of  oxygen,  I  will,  even  at  the 
risk  of  a  little  recapitulation,  go  through  the  calcnla- 


PRECAUTIONS.  195 

tion  :  A  molecule  of  KC1O3  weighs  39.1  +  35.5  +  48  = 
122.6  m.c.,  and  two  molecules  will  weigh  245.2  m.c. 
These  yield  2KC1,  weighing  2  (39.1  +  35.5)  =  149.2  m.c., 
and  3OO,  weighing  96  m.c.  We  must  next  find  the 
weight  of  ten  litres  of  oxygen  gas.  To  find  the  weight 
of  one  litre  we  multiply  the  specific  gravity  of  the  gas, 
or  half  molecular  weight,  by  T-|¥.  Now,  yf^x  16  = 
1.44  gramme.  Hence,  ten  litres  weigh  14.4  grammes. 
But,  if  96  m.c.  of  gas  are  made  from  245.2  m.c.  of 
salt,  then  14.4  grammes  would  be  obtained  from  a 
quantity  easily  found  from  the  proportion  : 

96  :  245.2  =  14.4  :  x  =  36.78  grammes. 

I  think,  after  this,  we  will  assume  that  these  quan- 
titative relations  are  all  right,  and  let  them  take  care 
of  themselves.  Returning  to  the  experiment,  before 
I  show  that  the  products  are  those  which  I  have  de- 
scribed, let  me  give  just  a  word  of  caution  to  any  of 
my  young  friends  present,  who  may  like  to  repeat  it. 

We  find  that  it  is  best  to  mix  our  chlorate  with  a 
heavy  black  powder,  known  in  commerce  as  black  ox- 
ide of  manganese.  What  the  effect  of  the  powder  is 
we  do  not  know,  for  it  is  wholly  unchanged  in  the 
process.  But,  in  some  way  or  other,  it  eases  off 
the  decomposition,  which  is  otherwise  apt  to  be  vio- 
lent. In  buying  the  black  oxide  of  manganese  you 
must  take  care  that  it  has  not  been  adulterated  with 
coal-dust — for  a  mixture  of  coal-dust  and  chlorate  ex- 
plodes with  dangerous  violence  when  heated,  and  seri- 
ous accidents  have  resulted  from  the  cupidity  which  led 
to  such  adulteration.  Let  me,  moreover,  say  in  general 
that,  although  I  highly  approve  of  chemical  experi- 
ments, as  a  recreation  for  boys,  they  ought  always  to  be 
made  under  proper  oversight,  and  according  to  exact 


196  CHEMICAL   CHANGES   CLASSIFIED. 

directions,  and  I  would  warmly  recommend,  as  a  trust- 
worthy companion  for  all  beginners,  the  abridgment  of 
"  Eliot  and  Storer's  Manual  of  Chemistry,"  recently 
edited  by  Prof.  Nichols,  of  the  Institute  of  Technol- 
ogy- 

But  how  shall  I  show  you  that  this  gas  we  have 
obtained  is  oxygen  ?  1  know  of  no  better  way  than 
to  test  it  with  one  of  our  watch-spring  matches.  ...  In 
no  other  gas  will  iron  burn  like  this. 

So  much  for  the  oxygen.  Let  us  next  turn  to  the 
other  product,  that  I  called  potassic  chloride.  This  is 
left  in  the  retort,  forming  a  solid  residue,  but,  as  it 
would  take  a  long  time  to  bring  what  we  have  just 
made  into  a  presentable  condition,  we  must  be  content 
to  see  some  of  the  product  of  a  former  process,  which 
I  have  in  this  bottle. 

At  a  distance,  you  cannot  distinguish  the  white  salt 
from  the  potassic  chlorate  with  which  we  started,  but, 
if  you  compared  the  two  carefully,  you  would  see  that 
there  was  a  very  great  difference  between  them.  I 
can  only  show  you  that  the  crystals  of  the  two  salts 
have  wholly  different  forms.  For  this  purpose  I  have 
crystallized  them  on  separate  glass  plates,  and  I  will 
now  project  a  magnified  image  of  the  crystals  on  the 
screen.  There  you  see  them  beautifully  exhibited  on  the 
two  illuminated  disks  side  by  side.  The  square  figures 
on  the  left-hand  disk  (Fig.  23)  are  the  projections  of 
the  cubes  of  potassic  chloride,  which  differ  utterly  in 
form  from  the  rhombic  plates  of  potassic  chlorate  that 
appear  on  the  right  (Fig.  24). 

The  second  example  of  an  analytical  process  which 
I  have  to  show  you  is  also  familiar  to  many  of  my 
audience,  and  cannot  fail  to  be  interesting  to  the  rest ; 
for  it  is  the  process  by  which  nitrous  oxide  is  prepared. 


PREPARATION   OF  NITROUS   OXIDE. 


197 


the  gas  now  so  much  used  by  the  dentists  as  an  anaes- 
thetic. It  was  formerly  called  laughing-gas,  but  the 
peculiar  intoxication  it  causes,  when  inhaled  under  cer- 
tain conditions,  has  been  almost  forgotten  in  its  present 


FIG.  2£.  -  Crystals  of  Potasgic  Chloride.          FIG.  24.— Crystals  of  Potassic  Chlorate. 

beneficent  application  in  minor  surgery.  Nitrous  oxide 
is  made  from  a  w^ell -known  white  salt,  prepared  from 
one  of  the  secondary  products  of  the  gas-works,  and 
called  nitrate  of  ammonia,  or  ammonic  nitrate.  "When 
this  salt  is  gently  heated  in  a  glass  flask,  its  molecules 
split  up  into  those  of  nitrous  oxide  and  water. 

Again,  let  us  make  use  of  the  time  required  for  the 
experiment  to  explain  the  process.  The  molecules  of 
ammouic  nitrate  have  the  constitution  N2H4O3,  and 
the  change  may  be  represented  thus  : 


N2H403 

Ammonic  Nitrate. 


2HaO 

Water. 


Nitrous  Oxide. 


The  experiment  has  been  arranged  so  as  to  show  both 
of  the  products  (Fig.  25).  The  water  condenses  in  this 
test-tube,  while  the  gas  passes  forward,  and  is  collected 
over  a  pneumatic  trough.  But  what  evidence  can  I 
give  you  that  these  are,  in  fact,  the  products  ?  As  re- 
gards the  water,  you  would  readily  recognize  the  fa- 


198  CHEMICAL   CHANGES  CLASSIFIED. 

miliar  liquid,  which  has  collected  in  the  tube,  could 
you  examine  and  taste  it.  But,  as  I  cannot  offer  you 
this  evidence,  I  will  seek  for  another.  Most  of  you 
must  be  familiar  with  the  remarkable  action  of  the 
alkaline  metals  on  water.  You  see  how  this  lump  of 
potassium  inflames  the  moment  it  touches  the  liquid. 


FIG.  25.— Preparation  of  Nitrous  Oxide  and  Water,  from  Ammonic  Nitrate. 

Let  us  now  see  whether  it  will  act  in  a  similar  way  on 
the  liquid  which  has  condensed  in  our  tube.  .  .  .  There 
can  be  no  doubt  that  we  are  dealing  with  water.  Next 
for  the  gas.  Nitrous  oxide  has  the  remarkable  quality, 
not  only  of  producing  anaesthesia,  but  also  of  sustain- 
ing the  combustion  of  ordinary  combustibles  with  great 
brilliancy — like  oxygen  gas.  But  there  is  a  marked 
difference  between  nitrous  oxide  and  oxygen,  which 
an  experiment  will  serve  to  illustrate,  and  this,  at  the 
same  time,  will  show  us  that  the  gas  we  have  obtained 
in  our  experiment  is  really  nitrous  oxide. 

Taking  a  lump  of  sulphur,  I  will,  in  the  first  place, 
ignite  it,  and  when  it  is  only  burning  at  a  few  points 
I  will  immerse  it  in  a  jar  of  oxygen.  As  you  see,  it  at 
once  burns  up  with  great  brilliancy.  Taking  now  a  sim- 


ANALYSIS   OF  NITROUS  OXIDE.  199 

ilar  lump  of  sulphur,  and  waiting  until  you  all  admit 
that  it  is  ignited  more  fully  than  before,  I  will  plunge 
it  into  this  jar  of  gas  we  have  just  prepared,  and  which 
we  assume  to  be  nitrous  oxide.  ...  It  at  once  goes 
out,  and  the  reason  is  obvious.  There  is  an  abundance 
of  oxygen  in  the  nitrous  oxide — relatively,  more  than* 
twice  as  much  as  in  the  air ;  but,  in  the  molecules  of 
JST2O,  the  oxygen  atoms  are  bound  to  the  atoms  of  ni- 
trogen by  a  certain  force,  which  the  sulphur  at  this 
temperature  is  unable  to  overcome.  Let  me,  however, 
heat  the  sulphur  to  a  still  higher  temperature,  until  the 
whole  surface  is  burning,  and  you  see  that  it  burns  as 
brilliantly  in  -the  compound  as  it  does  in  the  element- 
ary gas. 

In  the  experiment  with  ammonic  nitrate,  this  salt  is 
resolved,  not  into  elementary  substances,  but  only  into 
simpler  compounds,  and  it  will  be  instructive  to  inquire 
how  we  can  push  our  chemical  analysis  still  further, 
and  from  the  two  products — water  and  nitrous  oxide — 
obtain  the  elementary  substances  of  which  ammonic 
nitrate  is  ultimately  composed.  We  have  already  seen 
that,  by  an  electric  current,  water  may  be  changed  into 
two  elementary  aeriform  substances  named  oxygen  and 
hydrogen,  and  under  such  conditions  as  to  prove  that 
water  consists  of  these  two  chemical  elements,  and  of 
these  alone.  The  demonstration  having  once  been  given, 
it  is  unnecessary  to  repeat  this  simple  analytical  process  ; 
for  in  chemistry,  as  in  geometry,  we  should  make  little 
progress  if  we  were  always  retracing  our  first  steps. 
Passing,  then,  at  once  to  the  nitrous-oxide  gas,  let  me 
call  your  attention  to  two  successive  reactions  which, 
although  they  liberate  only  one  of  the  constituents  of 
this  compound,  clearly  point  out  what  the  other  con- 
stituent is,  and  enable  us  to  estimate  its  amount. 


200  CHEMICAL   CHANGES   CLASSIFIED. 

Heated  by  this  gas-furnace  is  a  glass  tube,  filled 
through  the  greater  part  of  its  length  with  finely-di- 
vided metallic  copper.  One  end  of  the  tube  is  con- 
nected by  air-tight  joints  with  a  graduated  gasometer, 
from  which  a  regulated  amount  of  nitrous-oxide  gas 
can  be  passed  through  the  interstices  left  by  the  copper. 
The  other  end  of  the  tube  is  connected  with  a  pneu- 
matic trough,  and  a  graduated  glass  bell  receives  the 
gas  after  it  has  passed  over  the  heated  metal.  While  I 
have  been  speaking,  the  gas-current  has  been  slowly 
passing,  and  the  aeriform  product  bubbling  up  through 
the  water  of  the  pneumatic  trough  into  the  bell.  A 
perfectly  colorless  gas  has  been  flowing  in  at  one  end  of 
the  tube,  and  an  equally  colorless  gas  passing  out  at  the 
other  end.  Moreover,  on  comparing  the  graduations  of 
the  gasometer  and  the  bell-glass,  I  find  that  there  has 
been  no  change  of  volume.  The  graduations  are  in 
French  measure,  and  just  750  cubic  centimetres  of  gas 
have  been  collected  in  the  bell ;  and  the  scale  shows 
that  just  750  cubic  centimetres  of  gas  have  left  the  gas- 
ometer. Has  not,  then,  the  gas  passed  over  the  metal- 
lic copper  unchanged  ?  A  careless  observer,  without 
testing  the  product,  might  so  conclude,  although  there 
is  one  circumstance  of  this  very  experiment  which,  if 
he  noticed  it,  would  show  him  conclusively  that  a 
chemical  change  has  taken  place.  The  surface  of  the 
metallic  copper  has  lost  its  characteristic  color  and  be- 
come covered  with  a  black  powder.  Such  an  essential 
change  of  qualities  indicates  here,  as  elsewhere,  a  chemi- 
cal change,  and  in  this  change  it  is  most  probable  that 
the  nitrous-oxide  gas  has  concurred.  To  make  sure  of 
this,  let  us  test  the  aeriform  product.  Here  are  two 
jars  of  gas  of  equal  size.  The  left-hand  jar  has  been 
filled  with  nitrous  oxide ;  the  right-hand  jar  with  the 


ANALYSIS  OF  NITROUS  OXIDE.  201 

product  which  would  be  formed  by  passing  this  gas 
over  heated  copper,  and  we  shall  be  reminded  of  the 
fact  that  this  product  has  the  same  volume  as  the  aeri- 
form factor  of  the  reaction  by  the  equal  volumes  of  the 
two  gases  as  they  stand  before  us.  Are  they  the  same 
substance?  "We  immerse  a  burning  taper  in  the  ni- 
trous oxide,  and  it  burns  far  more  brilliantly  than  in 
the  air.  We  immerse  the  taper  in  the  other  gas,  and  it 
is  instantly  extinguished.  Evidently  we  have  here  a 
very  different  substance  from  nitrous  oxide ;  and  this 
inert  gas  which  will  not  only  extinguish  a  candle-flame, 
but  also  the  flame  of  burning  phosphorus,  and  is  in  all 
its  relations  singularly  inactive,  is  a  very  well-known 
substance  named  nitrogen,  one  of  the  best  known  of  the 
chemical  elements. 

Having  thus  extracted  from  nitrous  oxide  one  of  its 
constituents,  we  naturally  next  ask,  How  much  of  the 
original  material  does  the  nitrogen  represent  ?  And  since 
the  volume  of  the  nitrogen  gas  obtained  exactly  equals 
the  volume  of  the  nitrous  oxide  that  has  disappeared,  it 
is  obvious  that  an  answer  to  this  question  can  be  ob' 
tained  by  comparing  the  densities  of  the  two  aeriform 
substances.  Now,  while  nitrous  oxide  has  22  times  the 
density  of  hydrogen  gas,  the  density  of  nitrogen  gas  on 
the  same  scale  is  only  14.  Hence,  while  of  22  parts  by 
weight  of  nitrous  oxide  14  are  nitrogen,  there  are  8 
parts  in  22  of  the  material  still  to  be  accounted  for. 
What  has  become  of  it  ?  Quite  obviously  it  has  united 
with  the  copper,  and  hence  the  change  of  color  which 
the  metal  has  undergone.  If  we  had  weighed  the  glass 
tube  with  its  copper  filling  before  the  experiment,  and 
should  now  weigh  it  again,  we  should  find  that  it  had 
gained  in  weight  by  the  exact  amount  missing.  There 
can  be,  then,  no  question  where  the  lost  material  is, 
15 


202  CHEMICAL   CHANGES  CLASSIFIED. 

and  it  would  be  very  satisfactory  if  we  could  extract  it 
from  the  black  powder  and  examine  the  substances  as 
we  have  the  nitrogen  gas.  But,  although  this  precise  re- 
sult cannot  be  produced,  we  can  readily  pass  the  mate- 
rial into  another  state  of  combination  so  familiar  in  all 
its  relations  that  we  shall  require  no  further  evidence 
in  regard  to  the  matter. 

For  this  purpose  we  will  now  alter  the  connections 
of  our  tube,  passing  in  hydrogen  gas  at  one  end,  and 
connecting  the  exit  with  a  small  U-shaped  glass  re- 
ceiver, in  which  any  volatile  product  will  condense. 
As  the  hydrogen  gas  passes  over  the  heated  black  pow- 
der, I  see  the  color  and  lustre  of  the  metallic  copper  re- 
appear, and  at  the  same  time  drops  of  a  limpid,  colorless 
liquid  appear  in  the  condenser.  That  liquid  is  water, 
and  the  problem  is  solved.  To  form  water,  hydrogen 
must  have  united  with  oxygen.  The  material  which 
united  with  the  copper  to  form  the  black  powder  in  the 
first  reaction,  and  which  the  black  powder  has  lost  in 
this  second  reaction  must  be  that  familiar  elementary 
substance — oxygen — surrounding  us  in  the  atmosphere, 
and  with  which  we  are  already  so  well  acquainted.  The 
black  powder  is  called  oxide  of  copper,  and  the  two 
processes  are  simply  expressed  by  chemical  symbols, 
thus : 


(1.)  |N,0[  +  Cu  =  CuO  +  |_NaJ.     (2.)  CuO  +  H2  =  Cu  +  HaO. 

In  the  first  reaction  we  have  inclosed  the  symbols 
of  nitrous  oxide  and  nitrogen  gas  in  squares,  in  order 
to  make  prominent  a  signification  of  the  symbolical 
language  which,  although  implied  in  what  has  been  be- 
fore said,  is  so  important  that  it  demands  more  specific 
notice.  When  a  reaction  is  correctly  written,  all  sym- 
bols of  molecules  stand  for  the  same  volume  in  the 


EXPLOSION   OF  IODIDE   OF  NITROGEN".  203 

state  of  gas  or  vapor.  Hence,  reaction  No.  1  expresses 
the  fact  above  stated,  that  the  volume  of  the  nitrogen 
gas  formed  is  equal  to  the  volume  of  nitrous  oxide 
decomposed.  In  other  cases,  where  the  volumes  are 
not  equal,  the  coefficients  before  the  molecular  symbols 
indicate  what  the  relation  is. 

But,  it  may  be  asked,  Why  all  this  reasoning  to 
establish  facts  which  we  had  already  assumed,  and  which 
the  symbols  represent?  Simply  in  order  to  illustrate 
the  methods  and  logic  of  chemistry.  Our  symbols,  it 
must  be  remembered,  prove  nothing.  They  are  merely 
concise  modes  of  expressing  observed  facts,  and,  unless 
we  clearly  distinguish  between  the  facts  of  Nature  and 
the  system  by  which  they  are  classified,  we  shall  attain 
to  no  positive  knowledge. 

The  last  example  of  an  analytical  reaction,  which 
we  shall  have  time  to  examine,  is  furnished  by  a  re- 
markable compound  of  iodine  and  nitrogen,  called 
iodide  of  nitrogen.  Iodine  is  an  elementary  substance, 
resembling  chlorine,  which  is  extracted  from  sea-weed. 
It  is  a  very  volatile  solid,  and  gives  a  violet-colored  va- 
por, whence  its  name  from  the  Greek  word  ico&rjs.  When 
heated  gently  with  aqua  ammonia,  the  iodine  takes 
from  the  ammonia  a  portion  of  nitrogen,  and  forms 
with  it  a  very  explosive  compound  whose  molecule  has 
the  constitution  NI8.  We  have  prepared  a  small  quan- 
tity of  the  substance,  and  the  black  powder  is  now  rest- 
ing on  this  anvil,  wrapped  in  filtering-paper.  The 
slightest  friction  is  sufficient  to  determine  the  break- 
ing up  of  these  very  unstable  molecules,  and  the  de- 
composition of  the  compound  into  iodine  and  nitro- 
gen. A  mere  touch  with  a  hammer  is  followed  by  a 
loud  report,  when  you  notice  a  cloud  of  violet  vapor, 
which  indicates  that  the  iodine  has  been  set  free : 


204  CHEMICAL   CHANGES  CLASSIFIED. 

2NI3  =  MJ  +  8I-I. 

Iodide  of  Nitrogen.  Nitrogen  Gas.  Iodine-Vapor. 

In  this  case,  as  in  previous  examples,  the  atoms,  when 
liberated,  unite  in  pairs  to  form  molecules  of  nitrogen 
gas  on  the  one  side,  and  molecules  of  iodine-vapor  on 
the  other ;  and,  since  a  single  molecule  does  not  yield 
an  even  number  of  atoms  of  either  kind,  we  double 
the  symbols. 

A  striking  feature  of  this  reaction,  which  you  can- 
not fail  to  have  recognized,  is  this :  A  compound  is 
here  spontaneously  resolved  into  elementary  substances 
with  development  of  energy  ;  so  that  iodine  and  nitro- 
gen, instead  of  attracting  each  other,  as  the  elements 
of  a  compound  ought  to  do,  actually  repel  each  other 
with  great  violence.  This  appearance  of  anomaly,  how- 
ever, arises  solely  from  our  habit  of  regarding  elemen- 
tary substances  as  the  true  elements  of  chemistry.  The 
molecules  of  nitrogen  gas  and  iodine  vapor  are  com- 
posed of  mutually  attracting  atoms  as  truly  as  the  mole- 
cules of  any  compound,  only  they  are  formed  by  the 
union  of  atoms  of  the  same  kind.  The  atoms  of  nitro- 
gen have  an  attraction  for  the  atoms  of  iodine,  or  else  no 
•combination  between  the  two  would  be  possible,  but 
their  attraction  for  each  other  is  far  stronger,  and  the 
explosion  is  simply  the  effect  of  the  greater  force  as- 
serting its  supremacy. 

Reviewing  for  a  moment  the  analytical  reactions  we 
have  studied,  let  me  call  your  attention  to  the  difference 
in  the  ease  with  which  they  can  be  obtained.  To  de- 
compose water  we  employ  a  powerful  electric  current, 
and  the  reaction  involves  a  great  expenditure  of  energy. 
Potassic  chlorate  and  ammonic  nitrate  decompose  spon- 
taneously, but  not  violently,  as  soon  as  the  salts  are 
melted  and  the  temperature  slightly  raised  above  the 


SYNTHESIS  OF   AMMONIC   CHLORIDE.  205 

melting-point,  and  the  decomposition  is  attended  with 
a  certain  development  of  energy,  as  shown  by  the  liber- 
ation of  heat.  Iodide  of  nitrogen  violently  explodes  on 
the  least  touch,  developing  a  large  amount  of  energy. 
Evidently,  there  is  a  marked  distinction  between  com- 
pounds like  water,- which  require  a  certain  amount  of 
energy  to  decompose  them,  and  compounds  like  those 
on  which  we  have  experimented  in  this  lecture,  that 
decompose  spontaneously  with  liberation  of  energy. 
The  first  class  are  stable  compounds,  and  their  stability 
is  measured  by  the  power  required  to  decompose  them. 
The  last  class  are  unstable  compounds,  and  the  power 
they  manifest,  when  some  slight  cause  destroys  the  equi- 
librium on  which  their  existence  depends,  and  they  fall 
in  pieces,  is  the  measure  of  their  instability.  This,  how- 
ever, introduces  us  to  an  order  of  phenomena  which  can- 
not be  -intelligently  discussed  until  our  knowledge  of 
chemical  processes  has  been  enlarged.  I  must  therefore 
content  myself  for  the  time  being  with  simply  pointing 
out  a  distinction  on  which  I  shall  afterward  dwell,  and 
pass  on  to  some  examples  of  synthetical  reactions. 

One  of  the  most  striking  illustrations  of  the  direct 
union  of  two  substances,  to  form  a  third,  is  furnished  by 
the  action  of  ammonia  gas  on  hydrochloric- acid  gas. 
Without  entering  into  any  details  in  regard  to  the  pro- 
cesses by  which  these  two  aeriform  substances  are  pre- 
pared, let  it  be  sufficient  to  say  that,  in  the  glass  flask 
on  the  right-hand  side  of  this  apparatus  (Fig.  26),  are 
the  materials  for  making  hydrochloric  acid,  and  in  the 
similar  flask  on  the  left  those  for  making  ammonia. 
The  exit-tubes  from  these  flasks  deliver  the  two  gases 
into  this  large  glass  bell,  where  they  meet,  and  the 
chemical  reaction  takes  place.  The  reaction  is  very 
simple,  and  one  in  regard  to  which  we  have  no  doubt. 


206  CHEMICAL   CHANGES  CLASSIFIED. 

for  the  molecules  of  both  of  the  factors  have  been 
weighed  and  analyzed.     It  is  expressed  thus  : 
NH3  +  HC1  =  NH4C1. 

Ammonia  Gas.  Hydrochloric-Acid  Gas.  Ammonic  Chloride. 


FIG.  26.— Combination  of  Ammonia  and  Hydrochloric-Acid  Gases. 

As  you  see,  the  atoms  of  a  molecule  of  ammonia  unite 
with  those  of  a  molecule  of  hydrochloric  acid  to  form 
a  single  molecule  of  ammonic  chloride,  and,  although 
the  reaction  may  imply  the  breaking  up,  to  a  certain 
extent,  of  the  molecules  of  the  two  factors,  yet  the 
subsequent  synthesis  is  the  chief  feature.  Arnmonic 
chloride  is  a  solid,  and  the  sudden  production,  from 
two  invisible  gases,  of  the  white  particles  of  this  salt, 
which  fill  the  bell  with  a  dense  cloud,  is  a  very  strik- 
ing phenomenon. 

The  second  example  of  synthesis  I  have  chosen  is 
equally  striking.  Here,  also,  the  factors  of  the  reaction 
are  both  gases. 

The  lower  jar  (Fig.  27)  contains  a  gas  called  nitric 
oxide,  like  nitrous  oxide,  a  compound  of  oxygen  and 
nitrogen,  but  containing  a  relatively  larger  proportion 
of  oxygen.  Its  molecule  has  the  constitution  NO. 


CHEMICAL  CHANGES  CLASSIFIED.  207 

The  upper  jar  contains  oxygen,  and,  on  removing  the 
thin  glass  which  now  separates  the  two  gases,  you  no- 
tice an  instantaneous  change.  A  deep-red 
vapor  soon  fills  the  glass.  This  red  prod- 
uct is  still  another  compound  of  nitrogen 
and  oxygen,  called  nitric  peroxide,  whose 
symbol  is  NO2,  and  the  reaction  is  simply 

this : 

2NO  +  O=O        =        2NO2. 

Nitric  Oxide.  Nitric  Peroxide. 

Here  a  molecule  of  nitric  oxide  takes  only 
an  atom  of  oxygen,  and,  since  each  mole- 
cule of  oxygen  gas  consists  of  two  atoms,  it 
will  supply  the  need  of  two  molecules  of  NO. 
Since  the  two  factors  and  the  single  prod- 
uct of  this  process  are  all  gases,  the  reaction 
^Gbinatlin°™f  bef ore  us  is  well  adapted  to  illustrate  a 
iSdoiy^en  ^ac^  m  regar(l  to  our  symbols,  of  which  I 
have  already  once  before  spoken.  If,  in 
writing  reactions,  care  is  taken  that  each  term  shall 
always  represent  one  or  more  perfect  molecules,  so  far 
as  their  constitution  is  known — then  the  symbols  will 
always  indicate,  not  only  the  relative  weights,  but  also 
the  relative  volumes  of  the  several  factors  and  products 
when  in  the  state  of  gas.  That  this  must  be  the  case, 
you  will  see  when  you  remember  that  equal  volumes 
of  all  gases  under  the  same  conditions  have  the  same 
number  of  molecules,  and  hence  that  all  gas-molecules 
have  the  same  volume.  The  symbol  of  one  molecule  rep- 
resents what  we  will  call  a  unit  volume,  and  the  number 
of  these  unit  volumes  concerned  in  any  reaction  is  the 
same  as  the  number  of  molecules.  We  can  read  the 
reaction  before  us  thus  :  Two  volumes  of  nitric-oxide 
and  one  volume  of  oxygen  gas  yield  two  volumes  of 
nitric  peroxide. 


208  TINSEL  BURNT  IN  CHLORINE  GAS. 

Three  volumes,  therefore,  become  two.  If  this  is 
the  case,  there  must  be  a  partial  vacuum  in  the  jar, 
and,  on  opening  the  stop-cock,  you  hear  the  whistle 
which  the  current  of  air  produces  as  it  rushes  in  to  es- 
tablish an  equilibrium. 

We  come  now  to  still  another  example  of  a  syn- 
thetical reaction,  and,  to  illustrate  this,  the  apparatus 
before  you  has  been  prepared  (Fig.  2.). 
The  metallic  leaf  in  the  upper  of  the  two 
glass  jars  is  made  of  brass,  which  consists 
of  the  two  metals,  zinc  and  copper.  In 
the  lower  jar  we  have  chlorine  gas.  The 
air  has  been  exhausted  from  the  upper 
jar  by  a  pump,  and,  on  opening  the  stop- 
cock, the  chlorine  gas  will  rush  in  from 
the  lower  jar  to  take  its  place.  Chemical 
union  at  once  results,  and  notice  the  ap- 
pearance of  flame,  which  is  an  indication 
that  great  heat  is  produced  by  this  chemical 
change.  The  change  here  is  very  simple. 
The  atoms  of  chlorine  unite  directly  with  FIG.  ss.— union  of 

•  T  i      .LI         />       •  i^  Chlorine  with 

the  atoms  both  of  zinc  and  of  copper,    Tinsel. 
forming  two  compounds,  which  we  call 
respectively  zincic  chloride,  and  cupric  chloride.     One 
reaction  will  serve  for  both  metals,  as  the  two  are  sim- 
ilar, differing  only  in  the  symbols  of  the  metals.    Take 
copper — 

Cu        +        oi-Cl        =        OuOl.. 

Copper.  Chlorine  Gas.  Cupric  Chloride. 

In  studying  analytical  reactions,  we  have  already 
made  a  distinction  between  stable  compounds  like  water, 
whose  decomposition  involves  a  certain  expenditure  of 
energy,  and  unstable  compounds  like  iodide  of  nitrogen, 
which  decompose  spontaneously  with  manifestation  of 
energy  whenever  the  equilibrium  of  the  molecules  is 


UNION   OF   IODINE    WITH   PHOSPHORUS.  209 

disturbed.  As  synthesis  is  the  direct  reverse  of  analysis, 
we  should  naturally  expect  that  the  thermal  or  dynamical 
effects  would  be  reversed  in  these  two  opposite  modes 
of  chemical  change ;  and  that  while  in  the  production 
of  stable  compounds  heat  or  energy  would  be  set  free,  in 
the  production  of  unstable  compounds  heat  or  energy 
would  be  consumed.  Such  a  relation  does  in  fact  exist, 
and  is  in  harmony  with  the  general  principles  of  action 
and  reaction,  which  rule  throughout  Nature.  But  with 
all  the  power  at  our  command  the  synthesis  of  unstable 
compounds  can  only  be  secured  by  indirect  processes. 
To  build  up  these  unstable  structures  is  very  much 
like  building  card  houses,  and  implies  not  only  an  ex- 
penditure of  energy  but  also  skill  in  construction ;  and 
it  is  a  great  triumph  of  our  modern  chemical  science 
that  we  have  been  able  to  accomplish  the  synthesis  of 
so  many  bodies  of  this  class.  I  shall  endeavor  to  show 
how  this  has  been  accomplished  in  a  future  lecture. 
We  are  not,  however,  dealing  with  such  cases  in  this 
connection,  but  only  with  examples  of  direct  chemical 
union.  Such  direct  union,  like  the  springing  of  an  arma- 
ture to  a  magnet,  always  implies  the  manifestation  of 
energy  or  the  development  of  heat,  and  heat  has  been 
evolved  in  all  the  three  synthetical  reactions  we  have 
thus  far  studied ;  but  the  point  is  so  important  that  I 
will  make  another  experiment  in  order  to  illustrate  this 
feature  of  direct  synthetical  reactions  still  further. 

In  this  glass  I  have  placed  a  small  piece  of  phos- 
phorus, and  now  I  will  drop  upon  it  a  few  crystals  of 
iodine.  Direct  combination  between  the  phosphorus 
and  iodine  at  once  takes  place,  and  the  heat  developed 
by  this  union  is  sufficient  to  inflame  the  uncombined 
phosphorus  which  I  have  intentionally  added  in  excess. 
But  the  burning  of  the  phosphorus,  although  the  most 


210  PHOsrnoRus  BURNT  IN  AIR. 

conspicuous  feature,  must  not  divert  our  attention  from 
the  primary  effect  which  it  is  the  object  of  the  experi- 
ment to  illustrate. 

There  is  one  class  of  chemical  processes  in  which 
the  thermal  effects  are  so  great,  so  striking,  and  so  im- 
portant, as  to  subordinate  all  other  phenomena.  I  re- 
fer to  the  common  processes  of  combustion,  on  which 
we  depend  for  all  our  artificial  light  and  heat.  To 
these  processes  I  shall  next  ask  your  attention,  for,  al- 
though they  are  only  further  illustrations  of  the  princi- 
ple just  stated,  yet,  they  play  such  an  important  part  in 
Nature,  and  have  been  so  often  the  battle-ground  be- 
tween rival  chemical  theories,  that  they  demand  our 
separate  attention.  I  will  open  the  subject  by  burning 
in  the  air  a  piece  of  phosphorus. 

Before  this  intelligent  audience  it  is  surely  unneces- 
sary to  dwell  011  the  elementary  facts  connected  with 
the  class  of  phenomena  of  which  this  is  the  type.  It 
will  only  be  necessary  for  me  to  call  to  your  recollec- 
tion the  main  points,  and  then  to  pass  to  the  few  feat- 
ures which  I  desire  especially  to  illustrate.  In  regard 
to  the  main  points,  no  experiment  could  be  more  in- 
structive than  this.  This  large  glass  jar  is  filled  with 
the  same  atmospheric  air  in  which  we  live.  Of  this 
atmospheric  air  one-fifth  of  the  whole  material  consists 
of  molecules  of  oxygen  gas  in  a  perfectly  free  and  un- 
combined  condition  ;  for,  although  they  are  mixed  with 
molecules  of  nitrogen  gas,  in  the  proportion  of  four  to 
one,  and,  although  the  presence  of  this  great  mass  cf 
inert  material  greatly  mitigates  the  violence  of  our  or- 
dinary processes  of  burning,  it  does  not,  in  any  other  re- 
spect, alter  the  chemical  relations  of  the  oxygen  gas  to 
combustible  substances.  These  combustibles  are,  for 
the  most  part,  compounds  of  a  few  elements — carbon, 


PHOSPHORUS  BURNT  IN  AIR.  211 

hydrogen,  sulphur,  and  phosphorus — including  the  ele- 
mentary substances  themselves,  and  our  common  com- 
bustibles are  almost  exclusively  compounds  of  hydrogen 
and  carbon  only.  Their  peculiar  relations  to  the  atmos- 
phere depend  solely  on  the  fact  that  the  atoms  of  these 
bodies  attract  oxygen  atoms  with  exceeding  energy, 
and  it  is  only  necessary  to  excite  a  little  molecular  ac- 
tivity in  order  to  determine  chemical  union  between 
the  two.  This  union  is  a  direct  synthetical  reaction, 
and,  like  all  processes  of  that  class,  it  is  attended  with  * 
the  liberation  of  heat.  The  chief  feature  which  dis- 
tinguishes the  processes  of  burning  from  other  synthet- 
ical reactions  is  the  circumstance  that  the  heat  gen- 
erated during  the  combination  is  sufficient  to  produce 
ignition — in  other  words,  to  raise  the  temperature  of 
the  materials  present  to  that  point  at  which  they  be- 
come luminous,  and  the  brilliant  phenomena  which 
thus  result  tend  to  divert  the  attention  from  the  sim- 
ple chemical  change,  of  which  they  are  merely  the  out- 
ward manifestation.  In  the  case  of  our  ordinary  com- 
bustibles, the  real  nature  of  the  process  is  still  further 
obscured  by  the  additional  circumstance  that  the  prod- 
ucts of  the  burning — carbonic  dioxide  and  aqueous  va- 
por— are  invisible  gases,  which,  by  mixing  with  the 
atmosphere,  so  completely  escape  rude  observation  that 
their  existence  even  was  not  suspected  until  about  a 
century  ago,  when  carbonic  dioxide  was  first  discovered 
by  Dr.  Black.  Although  these  aeriform  products  neces- 
sarily contain  the  whole  material,  both  of  the  combus- 
tible and  of  the  oxygen  with  which  the  combustible  has 
combined,  there  is  a  seeming  annihilation  of  the  com- 
bustible, which  completely  deceived  the  earlier  chem- 
ists. In  the  case  before  us,  however,  the  product  of 
the  combustion  is  a  solid,  and  it  is  this  circumstance 


212  POINT   OF  IGNITION. 

which  makes  the  experiment  so  instructive.  Almost 
every  step  of  the  process  can  be  here  seen.  You  no- 
ticed that  we  lighted  the  phosphorus  in  order  to  start 
the  combustion — for  this  combustible,  like  every  other, 
must  be  heated  to  a  certain  definite  temperature  before 
it  bursts  into  flame.  This  temperature  is  usually  called 
the  point  of  ignition,  and  differs  greatly  for  different 
combustibles.  While  phosphorus  inflames  below  the 
temperature  of  boiling  water,  coal  and  similar  combus- 
tibles require  a  full  red  heat.  If,  as  our  modern  theory 
assumes,  increased  temperature  merely  means  an  in- 
creased velocity  of  molecular  motion,  the  explanation 
of  these  facts  would  seem  to  be  that  a  certain  intensity 
of  molecular  activity  is  necessary  in  order  to  bring  the 
molecules  of  oxygen  sufficiently  near  to  those  of  the 
combustible  to  enable  the  atoms  to  unite,  and  that  the 
point  of  ignition  is  simply  the  temperature  at  which 
the  requisite  molecular  momentum  is  attained.  But 
the  process  once  started  continues  of  itself,  for  it  is  a 
characteristic  of  those  substances  we  call  combustible 
that,  as  soon  as  a  part  of  the  body  is  inflamed,  the  heat 
developed  by  the  chemical  union  is  sufficient  to  main- 
tain the  temperature  of  the  adjacent  mass  at  the  igni- 
tion-point. 

Passing  next  to  the  chemical  process  itself,  nothing 
could  be  simpler  than  the  change  which  is  taking  place 
in  the  experiment  before  us.  It  is  an  example  of  di- 
rect synthesis.  This  white  powder  which  you  see 
falling  in  such  abundant  flakes  is  the  solid  smoke  of 
this  fire.  It  is  formed  by  the  union  of  the  phosphorus 
and  oxygen — two  atoms  of  phosphorus  uniting  with 
five  of  oxygen  to  form  a  molecule  of  this  solid,  which 
we  call  phosphoric  oxide,  and  whose  symbol  we  may 
write  thus,  P2O5. 


CALORIFIC   POWER. 


213 


But,  neither  the  conditions  of  the  burning  nor  the 
chemical  change  itself,  although  so  beautifully  illus- 
trated here,  are  nearly  so  prominent  facts  as  the  mani- 
festation of  light  and  heat,  which  attends  the  process ; 
and  these  brilliant  phenomena  wholly  engrossed  the 
attention  of  the  world  until  comparatively  recently,  and 
indeed  they  still  point  out  what  is  really  the  most  im- 
portant circumstance  connected  with  this  class  of  phe- 
nomena. The  union  of  combustible  bodies  with  oxy- 
gen is  attended  with  the  development  of  an  immense 
amount  of  energy,  which  takes  the  form  of  light  or 
heat,  as  the  case  may  be.  Moreover,  it  is  also  true  that 
the  amount  of  energy  thus  developed  depends  solely 
on  the  amount  of  combustible  burnt,  and  not  at  all  on 
the  circumstance  that  the  burning  is  rapid  or  slow. 
Thus,  in  the  case  before  us,  the  amount  of  heat  devel- 
oped by  the  burning  of  an  ounce  of  phosphorus  is  a 
perfectly  definite  quantity,  and  would  not  be  increased 
if  the  combustion  were  made  vastly  more  intense.  So 
it  is  with  other  combustibles.  The  table  before  you 
gives  the  amount  of  energy  developed  by  the  burning 
of  one  pound  of  several  of  the  more  common  combus- 

Calorific  Power  from  One  Pound  of  Each  Combustible. 


English 
Units  of  Heat. 

Foot-pounds. 

Hydrogen  

62  032 

47  888  400 

Marsh-gas  

23  513 

18  152  350 

defiant  gas  

21  344 

16  477  880 

Wood-charcoal  

14544 

11,228  000 

Alcohol  

12  931 

9  982  890 

Sulphur  

4,070 

3  141  886 

tibles,  estimated,  in  the  first  place,  in  our  common  units 
of  heat,  and,  in  the  second  place,  in  foot-pounds.  But, 
although  the  amount  of  energy  is  thus  constant,  de- 


214      PHOSPHORUS  BURNT  IN  OXYGEN  GAS. 

pending  solely  on  the  amount  of  the  combustible  burnt, 
the  brilliancy  of  the  effect  may  differ  immensely.  A 
striking  illustration  of  this  fact  I  can  readily  show  you. 

For  this  purpose  I  will  now  repeat  the  last  experi- 
ment, with  only  this  difference,  that,  instead  of  burning 
the  phosphorus  in  air,  I  will  burn  the  same  amount  as 
before  in  a  globe  filled  with  pure  oxygen.  We  shall, 
of  course,  expect  a  more  violent  action,  because,  there 
being  here  no  nitrogen-molecules,  there  are  five  times 
as  many  molecules  of  oxygen  in  the  same  space.  Hence, 
there  are  five  times  as  many  molecules  of  oxygen  in  con- 
tact with  the  phosphorus  at  once,  and  five  will  combine 
with  the  phosphorus  in  the  same  time  that  one  did  be- 
fore. But,  with  this  exception,  all  the  other  conditions 
of  the  two  experiments  are  identical.  We  have  the 
same  combustible,  and  the  same  amount  of  it  burnt. 
We  have,  therefore,  the  same  amount  of  energy  devel- 
oped, and  yet  how  different  the  effect !  Phosphorus 
burns  brightly  even  in  air,  but  here  we  have  vastly 
greater  brilliancy,  and  the  intensity  of  the  light  is 
blinding. 

What  is  the  cause  of  the  difference  ?  One  obvious 
explanation  will  occur  to  all :  The  energy  in  this  last 
experiment  has  been  concentrated.  Although  only  the 
same  amount  of  heat  is  produced  in  the  two  cases,  yet, 
in  the  last,  it  is  liberated  in  one  fifth  of  the  time,  and 
the  effect  is  proportionally  more  intense.  The  inten- 
sity of  the  effect  is  shown  simply  in  two  circumstances : 
first,  a  higher  temperature ;  and,  secondly,  a  more  brill- 
iant light.  Of  these,  the  first  is  fully  accounted  for  in 
the  explanation  just  suggested ;  for,  if  five  times  as 
much  heat  is  liberated  in  a  given  time,  it  must  neces- 
sarily raise  the  temperature  of  surrounding  bodies  to  a 
much  higher  degree.  I  need  not  go  beyond  your  famil- 


PHOSPHORUS  BURNT  IN  OXYGEN  GAS.  215 

iar  experience  to  establish  this  principle,  although  tem- 
perature is  a  complex  effect,  depending,  not  only  on  the 
amount  of  heat  liberated,  but  also  on  the  nature  of  the 
material  to  be  heated,  and  on  conditions  which  deter- 
mine the  rapidity  with  which  the  heat  is  dissipated. 
But  the  matter  of  the  light  is  not  so  obvious.  Why 
should  more  rapid  burning  be  attended  with  more  brill- 
iant light  ?  It  is  so  in  the  present  case ;  but  is  it  al- 
ways so  ?  We  can  best  answer  this  question  by  a  few 
experiments,  which  will  teach  us  what  are  the  condi- 
tions under  which  energy  takes  the  form  of  light ;  but 
these  experiments  we  must  reserve  until  the  next  lect- 
ure. 


LECTUEE  X. 

THE   THEORY    OF    COMBUSTION. 

As  our  last  hour  closed,  we  were  studying  the  phe- 
nomena of  combustion.  I  had  already  illustrated  the 
fact  that,  so  far  as  the  chemical  change  was  concerned, 
these  processes  were  examples  of  simple  synthesis,  con- 
sisting in  the  union  of  the  combustible  atoms  with  the 
oxygen  atoms  of  the  air,  and  that  the  sole  circumstance 
which  distinguished  these  processes  from  other  synthet- 
ical reactions  was  the  amount  of  energy  developed. 
There  were  three  points  to  which  I  directed  your  at- 
tention in  connection  with  this  subject :  1.  The  con- 
dition of  molecular  activity,  measured  by  the  tempera- 
ture or  point  of  ignition,  which  the  process  requires. 

2.  The   chemical   change   itself,  always    very  simple. 

3.  The  amount   of  energy  developed,  and  the  form 
of  its  manifestation.     This  last  point  is  the  phase  of 
these  phenomena  which  absorbs  the  attention  of  be- 
holders, and  the  one  which  we  have  chiefly  to  study.    I 
stated  in  the  last  lecture  that  the  amount  of  energy  de- 
veloped depended  solely  on  the  nature  and  amount  of 
the  combustible  burnt,  but  I  also  showed  that  both  the 
intensity  and  the  mode  of  manifestation  of  this  energy 
varied  very  greatly  with  the  circumstances  of  the  ex- 
periment.    The  intensity  of  the  action  we  traced  at 


HYDROGEN  GAS  BURNT   IN   AIR.  217 

once  to  the  rapidity  of  the  combustion,  but  the  condi- 
tions which  determine  whether  the  energy  developed 
shall  take  the  form  of  heat  or  light  we  have  still  to  in- 
vestigate, and  no  combustible  is  so  well  adapted  as 
hydrogen  gas  to  teach  us  what  we  seek  to  know. 

Here,  then,  we  have  a  burning  jet  of  hydrogen.  It 
is  not  best  for  me  to  describe,  in  this  connection,  either 
the  process  or  the  apparatus  by  which  this  elementary 
substance  is  made,  and  a  constant  supply  maintained 
at  the  burner,  as  I  wish  now  to  ask  your  attention  ex- 
clusively to  the  phenomena  attending  the  burning  of 
the  gas ;  and  let  me  point  out  to  you,  in  the  first  place, 
that  hydrogen  burns  with  a  very  well-marked  flame. 
The  flame  is  so  slightly  luminous  that  I  am  afraid  it 
cannot  be  seen  at  the  end  of  the  hall,  but  I  can  make 
it  visible  by  puffing  into  it  a  little  charcoal-powder. 

Now,  all  gases  burn  with  a  flame,  and  flame  is  sim- 
ply a  mass  of  gas  burning  on  its  exterior  surface.  As 
the  gas  issues  from  the  orifice  of  the  burner,  the  cur- 
rent pushes  aside  the  air,  and  a  mass  of  gas  rises  from 
the  jet.  If 'the  gas  is  lighted — that  is,  raised  to  the 
point  of  ignition — this  mass  begins  to  combine  with 
the  oxygen  atoms  of  the  air  at  the  surface  of  contact, 
and  the  size  of  the  flame  depends  on  the  rapidity  with 
which  the  gas  is  consumed  as  compared  w^ith  the  rapid- 
ity with  which  it  is  supplied.  By  regulating  the  sup- 
ply with  a  cock,  as  every  one  knows,  I  can  enlarge  or 
diminish  the  size  at  will. 

The  conical  form  of  a  quiet  flame  results  from  the 
circumstance  that  the  gas,  as  it  rises,  is  consumed,  and 
thus  the  burning  mass,  which  may  have  a  considerable 
diameter  near  the  orifice  of  the  jet,  rapidly  shrinks  to 
a  point  as  it  burns  in  ascending. 

But  we  must  not  spend  too  much  time  with  these 
16 


218  THE  THEORY   OF  COMBUSTION. 

details,  lest  we  should  lose  sight  of  the  chemical  phi- 
losophy, which  it  is  the  main  object  of  this  course  to 
illustrate.  The  chemical  change  here  is  even  more 
simple  than  in  the  experiment  with  phosphorus,  and 
consists  solely  in  a  direct  union  of  the  hydrogen  atoms 
of  the  gas  with  the  oxygen  atoms  of  the  air.  Indeed, 
in  another  connection,  we  studied  the  reaction  at  an 
early  stage  in  this  course  of  lectures ;  when,  in  order 
to  illustrate  the  characteristic  feature  of  chemical  combi- 
nation, we  exploded  a  mixture  of  hydrogen  and  oxygen 
gases.  The  reaction  obtained  under  those  conditions 
was  identical  with  that  here.  We  had  not  then  learned 
to  express  the  chemical  change  with  symbols ;  but  now  I 
may  venture  to  write  the  reaction  on  the  black-board  : 
2H-H  +  0=0  =  2H2O. 

Hydrogen  Gas.  Oxygen  Gas.  Steam. 

It  would  be  very  easy  to  show  you  that,  as  the  sym- 
bols indicate,  from  two  volumes  of  hydrogen,  and  one 
of  oxygen,  two  volumes  of  steam  are  formed  ;  but  the 
experiment  requires  a  great  deal  of  time,  and  the  re- 
sult could  not  readily  be  made  visible  to  this  audience. 
I  must  content  myself  with  proving  that  water  is  really 
produced  by  the  hydrogen  flame. 

The  apparatus  we  use  looks  complicated,  but  is, 
in  fact,  very  simple  (Fig.  29).  By  means  of  an  aspira- 
tor the  products  of  combustion  are  sucked  through  a 
long  glass  tube,  which  is  kept  cool  by  a  current  of  wa- 
ter in  a  jacket  outside.  The  flame  burns  under  the 
open  and  flaring  mouth  of  the  tube,  and  the  liquid, 
which  condenses,  drops  into  a  bottle  at  the  other  end. 

You  must  not  expect  that  any  considerable  amount 
of  water  can  be  produced  in  this  way.  In  the  union 
of  the  two  gases  to  liquid  water,  a  condensation  of 
1,800  times  takes  place,  so  that,  in  order  to  obtain  a 


PRODUCT   OF   BURNING   HYDROGEN. 


219 


quart  of  liquid  water,  we  must  burn  1,200  quarts  of 
hydrogen  gas,  and  take  from  the  air  600  quarts  of  pure 
oxygen  ;  and  this,  on  the  scale  of  our  experiment, 
would  be  a  very  slow  process.  We  have  here  obtained 
barely  an  ounce  of  liquid,  although  the  jet  has  been 
burning  for  more  than  an  hour.  In  order  to  show  that 
the  product  is  really  water,  I  will  apply  the  same  test 
I  used  in  a  former  experiment.  We  will  pour  the 
liquid  into  a  shallow  dish,  and  drop  upon  it  a  bit  of 
potassium.  .  .  .  The  hydrogen  -  flame,  which  at  once 
bursts  forth,  gives  the  evidence  we  seek. 


FIG.  29.—The  Synthesis  of  Water. 

Such,  then,  being  the  nature  of  the  chemical  pro- 
cess before  us,  let  me  pass  on  to  that  feature  of  this 
flame  which  is  at  once  the  most  conspicuous  and  the 
most  important  phase  of  the  phenomenon,  namely,  the 
development  of  energy.  Here,  again,  we  have  become 
acquainted  with  the  important  facts  bearing  on  this 
question.  In  a  previous  lecture  I  told  you  that,  in  the 
burning  of  a  pound  of  hydrogen,  sufficient  energy  was 
developed  to  raise  a  weight  of  47,888,400  pounds  to 
the  height  of  one  foot,  and  these  figures  are  included, 


220  THE   THEORY   OF   COMBUSTION. 

among  other  data  of  the  same  kind,  in  the  diagram 
still  before  you.  (See  page  213.)  I  also  endeavored  to 
impress  on  your  minds  the  magnitude  of  this  energy 
by  showing  that,  with  a  hydrogen-flame,  a  temperature 
can  be  obtained  at  which  steel  burns  like  tinder.  In 
that  experiment,  however,  the  energy  was  intensified 
to  a  far  greater  degree  than  in  the  flame  we  have  here; 
for,  although  this  flame  is  very  hot,  it  is  wholly  inade- 
quate to  produce  the  effects  you  before  witnessed.  The 
intensity  was  then  gained  just  as  in  our  experiment  with 
phosphorus,  by  burning  the  hydrogen  in  pure  oxygen, 
instead  of  air ;  and  you  remember  the  apparatus,  called 
the  compound  blow-pipe,  by  which  this  result  was  ob- 
tained. 

The  flame  of  the  blow-pipe  emits  a  pale-blue  light, 
but  is  so  slightly  luminous  that  it  can  hardly  be  seen 
at  any  distance  in  this  large  hall,  and  yet,  as  we  know, 
it  is  intensely  hot.  You  have  seen  how  steel  defla- 
grates before  it,  and  I  will  now  show  you  its  effect  on 
several  other  metals  (copper,  zinc,  silver,  and  lead). 
You  notice  that  they  all  burn  freely,  and  that  each  im- 
parts to  the  flame  a  characteristic  color,  and,  I  may  add, 
in  passing,  that  spectrum  analysis,  which  has  achieved 
such  great  results  during  the  last  few  years,  is  based  on 
these  chromatic  phenomena. 

But  the  experiments  you  have  just  seen,  although 
so  brilliant  and  instructive,  have  not  yet  given  us  much 
help  toward  the  solution  of  the  problem  we  proposed 
to  investigate,  viz.,  the  conditions  under  which  the  en- 
ergy of  combustion  is  manifested  in  the  form  of  light. 
They  have,  however,  helped  us  thus  far :  they  have 
shown  that  the  light  cannot  depend  upon  the  rapidity  of 
the  combustion  or  the  temperature  of  the  flame  alone, 
for  here  we  have  intense  energy  and  a  very  high  tern- 


ON   WHAT   DOES   LUMINOUS  POWER   DEPEND?        221 

perature  without  light.  Moreover,  they  have  presented 
us  with  a  phenomenon,  which  differs  from  that  we  wit- 
nessed at  the  close  of  the  last  lecture,  in  the  very  point 
we  are  investigating  :  phosphorus  burns  in  oxygen 
with  a  most  brilliant  light ;  hydrogen  burns  in  oxygen 
with  scarcely  any  light. 

Now,  it  is  evident  that  the  cause  of  the  light  must 
be  some  circumstance  of  the  first  experiment,  which 
does  not  exist  in  this,  and,  by  comparing  the  two  to- 
gether, we  may  hope  to  reach  a  definite  result.  At  first 
sight,  this  comparison  reveals  only  resemblances.  Both 
processes  consist  in  the  union  of  combustible  material 
with  oxygen.  In  the  one  case  it  is  the  atoms  of  phos- 
phorus, and  in  the  other  the  atoms  of  hydrogen,  which 
combine  with  the  atoms  of  the  oxygen  gas.  Otherwise 
the  chemical  change  is  the  same  in  both  cases,  and  we 
cannot  therefore  refer  the  light  to  any  difference  in  the 
process.  Again,  in  both  processes  a  very  large  amount 
of  energy  is  developed,  but,  so  far  as  there  is  any  differ- 
ence, that  difference  is  in  favor  of  the  hydrogen,  which 
gives  the  least  light.  So,  also,  in  both  processes,  a 
very  high  temperature  is  attained  ;  but  a  simple  calcu- 
lation will  show  that  the  temperature  of  the  hydrogen- 
flame  is  higher  than  that  of  the  phosphorus-flame,  and 
so  the  light  cannot  be  an  effect  solely  of  temperature. 
Can  it  be  that  the  difference  is  due  to  the  circumstance 
that  the  combustible  in  one  case  is  a  solid,  and  in  the 
other  a  gas  ?  Here,  at  least,  is  a  difference,  which 
gives  us  a  starting-point  in  our  investigation.  But  we 
shall  not  pursue  the  investigation  far  before  we  find 
that  this  difference  is  wholly  illusory.  It  will  appear 
that  phosphorus  is  a  very  volatile  solid,  and  that  it  is 
wholly  converted  into  vapor  before  burning ;  so  that, 
in  fact,  we  are  dealing  in  both  cases  with  burning  gas. 


222  THE   THEORY   OF  COMBUSTION. 

In  looking  round  for  other  differences  we  shall 
recognize  that  there  is  a  marked  difference  in  the 
products  of  the  two  processes.  The  product  in  one 
case  is  phosphoric  oxide,  and  in  the  other  case  water. 
Water  is  volatile,  and  is  evolved  in  the  state  of  vapor. 
Phosphoric  oxide  is  a  highly-fixed  solid,  and  condenses 
in  those  snow-like  flakes  which  you  saw  falling  in  the 
jar  at  the  last  lecture.  May  it  not  be  that  the  circum- 
stance that  the  product  in  the  one  case  is  a  solid,  and 
in  the  other  a  gas,  is  the  canse  of  the  difference  in  the 
light?  In  the  phosphorus  flame  there  are  solid  parti- 
cles of  phosphoric  oxide,  while  in  the  hydrogen-flame 
there  are  no  solid  particles  whatever.  Can  this  be  the 
cause  of  the  difference?  Here,  at  least,  is  another 
starting-point  for  our  investigation. 

An  obvious  mode  of  discovering  whether  there  is 
any  value  in  this  suggestion  is  to  introduce  non-vola- 
tile solid  matter  into  the  blow-pipe  flame,  and  observe 
whether  the  light  of  the  flame  is  affected  thereby.  The 
temperature  of  the  flame  is  so  high  that  there  are  but 
few  solids  which  are  sufficiently  fixed  for  our  experi- 
ment. One,  however,  which  is  admirably  adapted  for 
our  purpose,  is  at  hand,  and  that  is  lime.  In  order, 
then,  to  answer  the  question  that  has  been  raised,  let 
us  introduce  into  the  flame  a  bit  of  lime,  or,  what 
amounts  to  the  same  thing,  allow  the  flame  to  play 
against  a  cylinder  of  this  material.  (In  an  instant  the 
hall  is  most  brilliantly  illuminated.)  The  question  is 
answered,  and  there  is  no  plainer  answer  than  that- 
given  by  a  well-considered  experiment. 

And  here  let  me  ask  your  attention  to  the  method 
we  have  followed,  because  it  illustrates,  in  the  most 
striking  manner,  the  method  of  science.  When  we 
wish  to  discover  the  cause  of  an  effect  observed  in  any 


POINTS  ESTABLISHED.  223 

phenomenon,  we  begin  by  varying  the  conditions  of 
the  phenomenon  until  at  last  we  find  that  the  effect 
varies,  or  perhaps  even  disappears.  That  is,  we  try  a 
series  of  experiments,  varying  the  conditions  at  each 
trial,  until  at  last  we  succeed  in  eliminating  the  eHect. 
This  having  been  done,  we  next  compare  the  condi- 
tions under  which  the  effect  appears  and  those  under 
which  it  does  not.  Those  conditions  common  to  both 
experiments  are  at  once  eliminated,  while  those  which 
are  different  in  the  two  are  carefully  considered,  and 
experiments  are  devised  to  test  their  influence  on  the 
effect  until  at  last  the  cause  is  made  evident.  Thus 
we  sought  to  find  the  cause  of  the  light  generally  pro- 
duced by  combustion.  We  began  by  burning  different 
combustibles  until  we  found  one  which  gave  out  little 
or  no  light.  We  next  compared  the  burning  of  phos- 
phorus in  oxygen,  which  gave  a  very  intense  light, 
with  the  burning  of  hydrogen,  which  gave  little  or 
none.  We  found  that  the  only  important  difference 
between  the  two  cases  was  the  circumstance  that  the 
phosphorus-flame  contained  particles  of  solid  matter, 
while  the  hydrogen-flame  contained  none,  and  in  order 
to  test  the  effect  of  the  difference,  which  the  compari- 
son suggested,  we  placed  solid  matter  in  the  hydrogen- 
flame,  when  the  cause  of  the  light  became  evident. 
This  method  of  comparing  phenomena  as  a  means  of 
discovering  the  cause  of  effects  which  are  prominent  in 
one,  although  common  to  both,  is  frequently  called 
differentiation,  and  it  is  one  of  the  most  valuable 
methods  of  science.  If  I  have  succeeded  in  giving 
you  some  idea  of  the  method,  the  time  we  have  de- 
voted to  these  experiments  has  been  well  spent. 

You  will  grant,  I  think,  that  we  have  now  established 
the  following  points  in  regard  to  the  theory  of  com  bus- 


224:  THE   THEORY   OF   COMBUSTION. 

tion :  1.  That  the  process  requires  a  certain  degree  of 
molecular  activity,  measured  roughly  by  what  we  call 
the  point  of  ignition.  2.  That  the  chemical  change 
consists  simply  in  the  union  of  the  combustible  with 
the  oxygen  of  the  air.  3.  That  these  processes  differ 
from  other  examples  of  synthesis  chiefly  in  the  circum- 
stance that  the  union  of  the  oxygen  atoms  with  those 
of  our  ordinary  combustibles  is  attended  with  an 
extraordinary  development  of  energy.  4.  That  the 
amount  of  this  energy  is  constant  for  the  same  com- 
bustible, and  is  in  each  case  exactly  proportional  to 
the  amount  of  fuel  burnt.  5.  That  the  intensity  of 
the  effect  depends  on  the  rapidity  of  the  combustion, 
the  energy  usually  manifesting  itself  as  heat,  but  tak- 
ing also  the  form  of  light  when  non-volatile  solid  parti- 
cles are  present.1 

Were  we  to  limit  our  regards  solely  to  the  theory 
of  combustion,  there  would  be  no  necessity  of  pursu- 
ing the  subject  further;  but  additional  experiments 
may  be  of  value  by  helping  you  to  associate  these 
principles  with  your  previous  experience.  To  this  end 
I  propose  to  ask  your  attention  to  the  burning  of  one 
of  the  most  familiar  combustibles,  viz.,  carbon  in  the 
form  of  charcoal,  and,  in  order  to  hasten  the  process, 
we  will  burn  the  charcoal  in  oxygen  gas  instead  of  air. 
Placing,  then,  a  few  lumps  of  charcoal,  previously  ignit- 
ed, in  a  deflagrating  spoon,  I  will  introduce  them  into 
this  large  jar  of  oxygen  gas.  ...  As  you  see,  the  char- 
coal burns  more  brilliantly  than  in  air.  But  even  in 
the  pure  gas  the  burning  is  by  no  means  very  rapid, 
and  the  reason  is  obvious.  Since  carbon,  in  all  its 

1  In  order  to  give  a  complete  view  of  the  subject,  it  would  be  necessary 
to  show  further  that  liquids,  and  even  vapors,  under  certain  conditions, 
may  become  brilliant  sources  of  light. 


CHARCOAL  BURNT  IN  OXYGEN  GAS.       225 

forms,  is  non-volatile,  the  molecules  of  the  charcoal 
cannot  leave  the  solid  lumps.  They  do  not,  therefore, 
go  half-way  to  meet  the  oxygen-molecules,  but  simply 
receive  those  which  are  driven  against  the  surface  of 
the  coals.  Hence  the  process  depends  on  the  activity 
of  the  oxygen-molecules  alone,  and,  since  the  number 
of  these  molecules  which  can  reach  the  combustible  in 
a  given  time  is  limited  by  the  extent  of  its  surface,  it 
is  evident  that  with  these  lumps  of  coal  we  cannot 
expect  very  rapid  burning  even  in  pure  oxygen.  If, 
however,  our  theory  is  correct,  we  should  greatly  in- 
crease the  rapidity  by  breaking  up  the  lumps,  and  thus 
increasing  the  surface  of  contact  with  the  gas.  Let  us 
see  if  the  result  answers  our  expectations. 

Taking,  then,  some  finely  -  pulverized  charcoal, 
already  ignited  (by  heating  the  mass  in  an  iron  dish 
over  a  spirit-lamp),  I  will  sift  the  red-hot  powder  from 
an  iron  spoon  into  another  large  jar  filled  with  oxy- 
gen. .  .  .  Nothing  we  have  yet  seen  has  exceeded  the 
splendor  of  the  chemical  action  which  now  results. 
This  dazzling  light  is  radiated  by  the  glowing  particles 
of  charcoal,  which,  after  they  have  become  incandes- 
cent, retain  their  solid  condition  until  the  last  atom  of 
carbon  is  consumed,  giving  us  another  illustration  of 
the  influence  of  this  circumstance  on  the  light :  and 
let  me  again  call  your  attention  to  the  great  fixity  of 
carbon  which  the  experiment  also  illustrates,  and  you 
will  at  once  recognize  the  importance  of  this  quality 
of  the  elementary  substance  in  localizing  our  fires,  as 
well  as  limiting  their  intensity,  and  will  see  that  the 
use  of  coal  as  fuel  wholly  depends  upon  it. 

Turn  next  to  the  chemical  change  itself.  This,  as 
in  the  other  similar  processes  we  have  studied,  is  an 
example  of  simple  synthesis,  consisting  in  the  union 


226  THE   THEORY  OF  COMBUSTION. 

of  the  carbon  atoms  with  oxygen.  As  to  the  nature 
of  the  product  formed,  a  single  experiment  will  give 
you  all  the  information  you  desire. 

After  removing  the  deflagrating  spoon  with  the 
residue  of  the  charcoal  lumps  from  the  first  of  the 
two  jars,  I  will  ask  you  to  notice  the  fact  that  the 
atmosphere  within  remains  as  transparent  as  before. 
The  eye  can  detect  no  evidence  of  change,  yet  all  the 
charcoal  that  has  disappeared  has  been  taken  up  by 
this  atmosphere,  and,  could  we  readily  weigh  the  mass 
of  gas,  I  could  show  you  that  the  weight  had  been  in- 
creased by  the  exact  weight  of  the  coal  absorbed.  In- 
deed, the  density  has  been  so  greatly  enhanced  that  I 
can  pour  the  gas  from  one  vessel  to  another  very  much 
as  I  would  water.  Let  me  pour  some  of  it  from  the 
jar  into  a  tall  glass  half  filled  already  with  lime  water. 
...  It  looks  like  child's-play;  but  the  transfer  has 
been  made,  and  now,  on  shaking  the  gas  and  lime- 
water  together,  the  liquid  becomes  milky. 

You  at  once  recognize  the  product :  chalk  has  been 
formed  in  the  lime-water,  and  the  gas  left  after  the 
burning  ceased  in  the  jar  must  be  the  same  carbonic 
dioxide  we  have  previously  studied.  We  made  the 
analysis  of  this  aeriform  substance  in  a  previous  lect- 
ure, and  we  have  now  made  the  synthesis.  See  how 
simply  we  express  the  reaction  : 

0        +        0=0        =        C02. 

Coal.  Oxygen  Gas.         Carbonic  Dioxide. 

A  fact  is  indicated  by  this  reaction,  which  we  must 
not  overlook.  The  volume  of  the  carbonic  dioxide 
(CO2)  obtained  is  exactly  equal  to  the  volume  of  the 
oxygen  gas  (O=O)  employed.  In  this  experiment  we 
used  a  jarful  of  oxygen  and  we  obtained  a  jarful  of 
carbonic  dioxide.  The  material  of  the  burnt  charcoal 


ENERGY  DEVELOPED.  227 

is  taken  up  into  the  gas  atom  by  atom,  actually  ab- 
sorbed by  it  as  a  sponge  absorbs  water.  Every  mole- 
cule of  oxygen  which  strikes  against  the  charcoal  flies 
off  with  an  atom  of  carbon,  forming  with  it  the  mole- 
cule of  carbonic  dioxide  which,  of  course,  occupies  the 
same  space  as  the  previous  molecule  of  oxygen  gas. 
Hence  it  is  that  the  vast  amount  of  carbon  which  is 
being  constantly  absorbed  by  the  atmosphere,  as  it 
passes  through  our  grates  and  furnaces,  does  not  alter 
its  volume.  Would  that  I  might  impress  this  re- 
markable fact  on  your  imagination !  Consider  how 
much  coal  is  being  burnt  every  day  in  a  city  like  this — 
hundreds  and  hundreds  of  tons  !  Conceive  of  what  a 
mass  it  would  make,  more  than  filling  this  large  hall 
from  floor  to  ceiling,  and  yet  in  our  city  alone  this 
enormous  black  mass  is  in  twenty-four  hours  absorbed 
by  the  transparent  air,  picked  up  and  carried  away 
bodily,  atom  by  atom,  by  the  oxygen -molecules. 

Turn  now  to  the  energy  developed  in  this  process. 
Our  diagram  indicates  that  the  amount  of  energy  de- 
veloped by  the  burning  of  a  pound  of  coal  is  very 
much  less  than  that  obtained  with  a  pound  of  hydro- 
gen. But  then  it  must  be  remembered  how  attenuated 
hydrogen  gas  is ;  if,  instead  of  comparing  equal  weights, 
we  compare  equal  volumes,  we  shall  find  that  the  differ- 
ence is  vastly  in  favor  of  carbon. 

Most  of  the  combustible  materials,  however,  which 
we  use  as  fuel,  consist  of  both  hydrogen  and  carbon ; 
but  the  phenomena  we  have  studied  in  the  burning  of 
the  elementary  substances  reappear  with  these  familiar 
combustibles,  and,  in  regard  to  them,  there  are  only  a 
few  special  points  to  be  noticed.  On  many  of  these 
substances,  such  as  naphtha,  paraffine,  stearine,  wax,  oil, 
and  the  like,  the  effect  of  the  heat  is  to  generate  illu- 


228  THE   THEORY   OF   COMBUSTION. 

minating  gas,  which  is  the  source  of  most  of  our  arti- 
ficial light.  In  our  cities  and  large  towns  the  gas  is 
made  for  us  by  a  special  process,  but  it  must  be  remem- 
bered that  every  lamp  and  candle  is  a  small  gas-fac- 
tory. Flame  is  always  burning  gas,  and  the  gas 
wrhich  we  burn  in  our  lamps  and  candles  is  very  similar 
to  that  supplied  by  the  Boston  Gas  Company  :  the  only 
difference  is  that  the  gas,  instead  of  being  made  from 
bituminous  coal,  is  made  from  petroleum  or  wax,  and, 
instead  of  being  made  at  the  "North  End"  and  dis- 
tributed through  pipes  to  distant  burners,  is  burnt  as 
fast  as  it  is  made.  The  heat  generated  by  the  burning 
gas  is  so  great  that  it  volatilizes  the  oil  or  wax  fast 
enough  to  supply  the  flame,  and  then  the  mechanism 
of  the  wick  comes  into  play  to  keep  the  parts  of  these 
natural  gas  machines  in  perfect  running  order.  In- 
deed, a  common  candle,  simple  as  it  appears  to  be,  is 
a  most  wonderful  apparatus,  and  I  should  be  glad  to 
occupy  the  whole  hour  in  explaining  the  adaptation  of 
its  parts ;  but  I  have  only  time  for  a  few  illustrations, 
which  show  that  in  these  luminous  flames,  as  in  the 
other  cases  of  combustion  we  have  studied,  the  light 
comes  from  incandescent  solid  particles. 

Of  the  two  constituents  of  the  combustible  gas 
which  forms  the  flame,  hydrogen  is  the  most  combusti- 
ble, and  under  ordinary  conditions  is  the  first  to  burn, 
setting  free,  for  a  moment,  the  accompanying  carbon  in 
the  form  of  a  fine  soot  which  fills  the  light-giving  cone. 
This  dust  is  at  once  intensely  heated,  and  each  glowing 
particle  becomes  a  centre  of  radiation,  throwing  out  its 
luminous  pulsations  in  every  direction.  The  sparks 
last,  however,  but  an  instant,  for  the  next  moment  the 
charcoal  is  itself  consumed  by  the  fierce  oxygen,  now 
aroused  to  full  activity,  and  only  a  transparent  gas  rises 


THE  FAMILY  GAS-FACTORY.  229 

from  the  flame.  But  the  same  process  continues ;  other 
particles  succeed,  which  become  ignited  in  their  turn, 
and  hence,  although  the  sparks  are  evanescent,  the  light 
is  continuous. 

I  might  illustrate  this  theory  by  the  familiar  fact  that 
<>oot  is  at  once  emitted  from  all  these  luminous  flames, 
whenever  the  draft  becomes  so  far  interrupted  that  it 
does  not  supply  sufficient  oxygen  to  burn  completely 
the  carbon  particles ;  but  a  still  more  striking  illustration 
is  furnished  by  the  simple  contrivance  we  employ  in  the 
laboratory  for  preventing  the  deposition  of  this  soot  on 
the  heating  surfaces  of  our  chemical  vessels.  We  use 
for  this  purpose  a  gas-burner  invented  by  Prof.  Bunsen, 
of  Heidelberg,  and  known  by  his  name,  in  which  air  is 
mixed  with  the  hydrocarbon  gas  before  it  is  burnt. 
But  this  air,  while  it  prevents  the  formation  of  soot, 
at  the  same  time  destroys  the  illuminating  power  of  the 
flame.  The  molecules  ot  the  hydrocarbon  gas  being 
now  in  near  proximity  to  the  molecules  of  oxygen  re- 
quired for  complete  combustion,  the  difference  of  af- 
finity of  oxygen  for  the  carbon  and  hydrogen  atoms 
does  not  come  into  play.  There  is  enough  oxygen  for 
all,  and  the  result  is  that  no  carbon-particles  are  set 
free  in  the  flame.  We  have  no  soot,  and  therefore  no 
light. 

In  this  Bunsen  lamp  the  size  of  the  apertures,  by 
which  the  air  enters  at  the  base  of  the  burner,  may  be 
regulated  by  a  valve,  and  you  notice  that  on  closing 
this  valve  the  flame  at  once  becomes  luminous.  Open 
it  again  so  that  the  gas  shall  mix  with  air  before  burn- 
ing, and  the  energy  no  longer  takes  the  form  of  light. 
See,  nevertheless,  how  brightly  the  flame  ignites  this 
coil  of  platinum  wire,  showing  that  there  is  no  want 
of  energy,  only  it  now  appears  wholly  as  heat. 


230  THE   THEORY   OF   COMBUSTION. 

The  flame  of  a  wood  or  soft-coal  fire  is  also  a  gas- 
flame.  The  first  effect  of  heat  on  these  bodies  is  to 
generate  illuminating  gas,  and  to  this  circumstance,  as 
in  the  case  of  the  candle,  the  flame  is  due,  but  after  a 
while  all  the  hydrogen  is  driven  off,  and  we  have  then, 
in  the  glowing  embers,  the  flameless  combustion  of 
carbon. 

The  chemical  change  which  takes  place  in  the  burn- 
ing of  hydrocarbon  fuels  is  in  no  way  affected  by  the 
circumstance  that  the  hydrogen  and  carbon  are  in 
chemical  union.  All  the  hydrogen- atoms  burn  to 
water,  and  all  the  carbon-atoms  to  carbonic  dioxide, 
and  these  products  can  be  detected  in  the  smoke  of 
every  flame ;  indeed,  with  a  few  unimportant  excep- 
tions, they  are  the  sole  products  of  the  combustion. 

Take,  lor  example,  this  candle-flame.  On  holding 
over  it  a  cold  bell-glass  the  glass  soon  becomes  be- 
dewed, and,  before  long,  drops  of  water  begin  to  trickle 
down  the  sides ;  and  now,  on  inverting  the  bell,  and 
shaking  up  in  it  some  lime-water,  the  milky  appear- 
ance, which  the  clear  solution  immediately  assumes, 
indicates  the  presence  of  carbonic  dioxide. 

Of  course,  all  the  material  of  the  candle  passes  into 
these  colorless  and  insensible  aeriform  products  which 
mingle  with  the  atmosphere,  and  this  absorption  of 
combustible  material  into  the  atmosphere,  this  melting 
of  firm,  solid  masses  of  coal  and  wood  into  thin  air,  has 
such  an  appearance  of  annihilation  that  it  requires  all 
the  power  of  the  reason,  aided  by  experiment,  to  cor- 
rect the  false  impression  of  the  senses.  Yet  nothing 
is  easier  than  to  show  that  the  smoke,  colorless  and 
insensible  as  it  is,  weighs  more  than  the  material 
burnt,  and,  although  the  experiment  must  be  familiar 
to  many  of  my  audience,  I  will  repeat  it,  because  it 


NO  LOSS  OF  MATERIAL.  231 

may  aid  some  to  clearer  views  of  this  all-important 
subject. 

Let  me  call  your  attention,  then,  to  this  candle 
which,  in  a  candlestick  of  peculiar  construction,  is 
hanging  equipoised  from  one  end  of  the 
beam  of  this  balance  (Fig.  30).  You 
know  that  both  aqueous  vapor  and  car- 
bonic dioxide  are  eagerly  absorbed  by 
caustic  soda,  and  this  apparatus  is  so  ar- 
ranged that  the  smoke  of  the  candle  is 
sucked  through  two  glass  tubes  filled 
with  this  absorbent  material.  You  no- 
tice that  my  balance  is  in  equilibrium, 
and  I  will  now  light  the  candle  under  its 
FIG.  so.  tin  chimney.  The  products  of  the  com- 
bustion rise  to  the  top  of  the  chimney, 
which  is  closed  excepting  two  small  apertures,  through 
which  the  smoke  is  sucked  into  the  glass  tubes  contain- 
ing the  caustic  soda.  Now  you  must  picture  to  your- 
selves the  molecules  of  oxygen  of  our  atmosphere  rushing 
in  on  this  candle-flame  from  every  side,  each  one  seizing 
its  atom  of  carbon,  or  its  four  atoms  of  hydrogen,  as  the 
case  may  be.  You  must;  then,  follow  the  molecules  of 
carbonic  dioxide  and  water  thus  formed,  as  they  are 
caught  up  by  the  current  of  air — which  our  aspirator 
draws  through  the  apparatus  —  and  hurried  into  the 
glass  tubes,  where  they  are  seized  upon  and  held  fast 
by  the  caustic  soda.  All  the  smoke  of  the  candle  being 
thus  retained,  it  is  evident  that,  if  the  process  is  as  I 
have  described  it,  we  should  expect  that  the  apparatus 
would  increase  in  weight  as  the  candle  burns,  while,  on 
the  other  hand,  were  any  part  of  the  material  lost,  there 
would  be  a  corresponding  diminution  in  weight.  And 
we  not  only  find  that  the  weight  increases,  as  the  bal 


232  THE   THEORY   OF  COMBUSTION. 

ance  shows,  but  that  the  increase  is  exactly  equal  to 
the  amount  of  oxygen  consumed.  Not  only  none  of 
the  material  of  the  candle  escapes  from  the  apparatus, 
but  a  portion  of  the  oxygen  of  the  air  is  also  retained, 
arid  that  causes  the  increase  of  weight. 

In  connection  with  this  experiment,  I  must  not  fail 
to  call  your  attention  to  the  circumstance  that  the  prod- 
ucts of  this  combustion  are  as  harmless  as  they  are  im- 
perceptible to  the  senses.  Eemember  that  thousands 
of  tons  of  carbonic  dioxide  and  aqueous  vapor  are  dis- 
charged into  the  air  of  this  city  in  a  single  day.  Eemem- 
ber, also,  what  a  howl  of  remonstrance  goes  up  if,  from 
some  manufactory,  a  few  pounds  of  similar  but  noisome 
products  escape,  and  you  cannot  fail  to  recognize  the 
importance  of  this  fact  in  the  economy  of  Nature. 
Add  to  this  what  you  already  know,  that  the  smoke  of 
our  fires  and  the  exhalations  of  our  lungs  is  the  food 
of  the  plant — that  the  whole  vegetable  world  is  con- 
stantly absorbing  carbonic  dioxide,  and  giving  back  the 
oxygen  to  the  atmosphere  while  storing  up  the  regen- 
erated carbon  in  its  tissues,  and  you  will  be  still  further 
impressed  by  the  wonderful  revelations  we  are  study- 
ing. 

Nor  must  we,  in  this  connection,  fail  to  notice  again 
the  enormous  amount  of  energy  which  the  burning  of 
our  common  forms  of  fuel  liberates.  The  table  is  still 
before  you  which  shows  how  great  is  the  amount  of 
energy  which  can  be  obtained  by  the  burning  of  a  sin- 
gle pound  either  of  hydrogen  gas  or  of  charcoal,  and 
the  relations  of  these  elementary  substances  in  this  re^ 
spect  are  not  in  the  least  altered  by  their  association  in 
common  wood  or  coal.  In  round  numbers,  it  may  be 
said  that  a  cubic  foot  of  cannel  coal  contains  sufficient 
energy,  if  wholly  utilized,  to  raise  a  weight  of  3,269 


SOURCE  OF  THE  ENERGY.  233 

tons  one  hundred  feet,  or  732,000,000  pounds  one  foot. 
I  said,  if  wholly  utilized,  for,  although  we  are  able  to 
make  use  of  the  whole  energy  in  the  form  of  heat,  we 
have  not  yet  succeeded  in  applying  more  than  about 
one-twentieth  of  it  to  mechanical  work.  But  still  the 
energy  exists  stored  up  for  use  in  every  foot  of  wood  or 
coal,  and  is  ready  to  be  set  free  when  the  fuel  is  burnt. 

"When  standing  before  a  grand  conflagration,  wit- 
nessing the  display  of  mighty  energies  there  in  action, 
and  seeing  the  elements  rushing  into  combination  with 
a  force  which  no  human  energy  can  withstand,  does  it 
seem  as  if  any  power  could  undo  that  work  of  destruc- 
tion, and  rebuild  those  beams  and  rafters  which  are 
melting  into  air  ?  Yet,  in  a  few  years  thay  will  be  re- 
built. This  mighty  force  will  be  overcome  ;  not,  how- 
ever, as  we  might  expect,  amid  the  convulsions  of  Na- 
ture or  the  clashing  of  the  elements,  but  silently  in  a 
delicate  leaf  waving  in  the  sunshine.  As  I  have  al- 
ready explained,  the  sun's  rays  are  the  Ithuriel  wand, 
which  exerts  the  mighty  power,  and  under  the  direction 
of  that  unerring  Architect,  whom  all  true  science  rec- 
ognizes, the  woody  structure  will  be  rebuilt,  and  fresh 
energy  stored  away  to  be  used  or  wasted  in  some  future 
conflagration. 

My  friends,  this  is  no  theory,  but  sober,  well-estab- 
lished fact.  How  the  energy  comes  and  how  it  is  stored 
away,  we  attempt  to  explain  by  our  theories.  Let  these 
pass.  They  may  be  true,  they  may  be  mere  fancies  ; 
but,  that  the  energy  comes,  that  it  is  stored  away,  and 
that  it  does  reappear,  are  as  much  facts  as  any  phe- 
nomena which  the  sun's  rays  illuminate.  I  know  of  no 
facts  in  the  whole  realm  of  Nature  more  wonderful 
than  these,  and  I  return  to  them  in  the  annual  round 
of  my  instruction  with  increasing  wonder  and  admira- 
17 


234  THE  THEORY   OF   COMBUSTION. 

tion,  amazed  at  the  apparent  inefficiency  of  the  means, 
and  the  stupendous  magnitude  of  the  result.  In  an- 
other course  of  lectures  in  this  place  I  endeavored  to 
show  what  weighty  evidence  these  facts  give  in  support 
of  the  argument  that  all  the  details  have  been  arranged 
by  an  intelligent  Designer.1  The  plan  of  this  course 
does  not  give  me  time  to  do  more  than  allude  to  this 
point,  and  I  only  refer  to  it  here  to  ask  for  the  argu- 
ment your  own  careful  consideration. 

There  is  still  another  point,  in  connection  with  this 
subject,  to  which  also  I  can  only  barely  allude.  The 
crust  of  our  globe  consists  almost  wholly  of  burnt  ma- 
terial. Our  granite,  sandstone,  and  limestone  rocks, 
are  the  cinders  of  the  great  primeval  fire,  and  the  at- 
mosphere of  oxygen  the  residue  left  after  the  general 
conflagration — left  because  there  was  nothing  more  to 
burn.  Whatever  of  combustible  material,  wood,  coal, 
or  metal,  now  exists  on  the  surface  of  the  earth,  has 
been  recovered  from  the  wreck  of  the  first  conflagration 
by  the  action  of  the  sun's  rays.  One-half  of  all  known 
material  consists  of  oxygen,  and,  on  the  surface  of  the 
globe,  combination  with  oxygen  is  the  only  state  of 
rest.  In  the  process  of  vegetable  growth,  the  sun's 
rays  have  the  power  of  freeing  from  this  combination 
hydrogen  and  carbon  atoms,  and  from  these  are  formed 
the  numberless  substances  of  which  both  the  vegetable 
and  animal  organisms  consist.  From  the  material  of 
these  organisms  we  make  charcoal,  and  Nature  makes 
her  coal-beds,  and  supplies  her  petroleum -wells-  More- 
over, with  these  same  materials,  man  has  been  able  to 
separate  the  useful  metals  from  their  ores,  and,  by  the  aid 

1  "  Religion  and  Chemistry ;  or,  Proofs  of  God's  Plan  in  the  Atmos- 
phere and  its  Elements,"  ten  lectures  by  Josiah  P.  Cooke,  published  by 
Charles  Scribner.  New  York,  1880. 


SOURCE  OF  THE  ENERGY.  235 

of  various  chemical  processes,  to  isolate  the  other  ele- 
mentary substances  from  their  native  compounds ;  but 
the  efficiency  of  all  these  processes  depends  on  em- 
ploying the  energy  which  the  sun's  rays  impart  to  the 
carbon  and  hydrogen  atoms  to  do  work.  A  careful 
analysis  of  the  conditions  will  show  that  it  is  just  as 
truly  the  sun's  energy  which  parts  the  iron  from  its 
combination  in  the  ore,  as  it  is  solar  power  which  parts 
the  carbon  from  the  carbonic  dioxide  in  the  leaf.  We 
have  here,  however,  but  a  single  example  of  a  general 
truth.  All  terrestrial  energy  comes  from  the  sun,  and 
every  manifestation  of  power  on  the  earth  can  be 
traced  directly  back  to  his  energizing  and  life-giving 
rays.  The  force  with  which  oxygen  tends  to  unite  with 
the  other  elements  may  be  regarded  as  a  spring,  which 
the  sun's  rays  have  the  power  to  bend.  In  bending 
this  spring  they  do  a  certain  amount  of  work,  and, 
when,  in  the  process  of  combustion,  the  spring  flies 
back,  the  energy  reappears.  Moreover,  the  instability 
of  all  organized  forms  is  but  a  phase  of  the  same  action, 
and  the  various  processes  of  decay,  with  the  accompa- 
nying phenomenon  of  death,  are  simply  the  recoiling 
of  the  same  bent  spring.  Amid  all  these  varied  phe- 
nomena, the  one  element  which  reappears  in  all,  and 
frequently  wholly  engrosses  our  attention,  is  energy; 
and,  if  I  have  succeeded  in  fixing  your  attention  on  this 
point,  my  great  object  in  this  lecture  has  been  gained. 
In  the  early  part  of  this  course,  I  stated  that  all  modern 
chemistry  rests  on  the  great  truth  that  MATTER  is  INDE- 
STRUCTIBLE, AND  is  MEASURED  BY  WEIGHT.  This  evening 
we  have  seen  glimpses  of  another  great  central  truth, 
which,  although  more  recently  discovered,  is  not  less 
far-reaching  or  important,  namely,  ENERGY  is  INDE- 
STRUCTIBLE, AND  IS  MEASURED  BY  WORK.  Add  to  these 


236  THE  THEORY   OF   COMBUSTION. 

two  a  third,  namely — INTELLIGENCE  is  INDESTRUCTIBLE, 

AND   IS   MEASURED   BY   ADAPTATION and   yOU    have,  RS  it 

seems  to  me,  the  three  great  manifestations  of  Na- 
ture :  MATTER,  ENERGY,  and  INTELLIGENCE.  These  great 
truths  explain  and  supplement  each  other.  Give  to 
each  its  due  weight  in  your  philosophy,  and  you  will 
avoid  the  extremes  of  idealism  on  the  one  side,  and  of 
materialism  on  the  other. 


LECTUKE  XL 

GUNPOWDER   AND   NITROGLYCERINE. 

THERE  is  one  further  point  in  connection  with  the 
theory  of  combustion  to  which  I  wish  to  call  your  at- 
tention, at  the  outset  of  my  lecture  this  evening.  In 
the  only  cases  of  burning  we  have  studied,  the  combus- 
tible unites  with  the  oxygen  of  the  atmosphere.  It  is 
possible,  however,  to  have  combustion  without  atmos- 
pheric air,  the  combustible  obtaining  the  required 
oxygen  from  some  associated  substance.  There  are 
several  substances  in  which  a  large  amount  of  oxygen 
is  so  loosely  combined,  or,  in  other  words,  in  which  the 
oxygen-atoms  are  held  in  combination  by  such  a  fee- 
ble force,  that  they  will  furnish  oxygen  to  the  combus- 
tible as  readily  as  the  atmosphere,  and  in  a  vastly  more 
concentrated  form.  Two  of  these  substances  are  well 
known,  nitre  (potassic  nitrate)  and  chlorate  of  potash 
(potassic  chlorate).  One  ounce  of  this  last  salt-— the 
quantity  in  this  small  crucible — contains  enough  oxygen 
to  fill  a  large  jar  (1.7  gallon),  and  by  simply  heating 
the  salt  we  should  obtain  that  amount  of  oxygen  gas. 
We  have  provided  also  one-third  of  an  ounce  of  pul- 
verized sugar,  and  we  will  now  mix  the  two  powders 
thoroughly  together.  Consider  the  conditions  in  this 


238  GUNPOWDER. 

mixture :  The  sugar  is  a  combustible  substance,  and 
every  particle  of  this  combustible  is  in  contact  with,  or, 
I  should  rather  say,  in  close  proximity  to,  grains  of 
chlorate  of  potassa,  which  contain  sufficient  oxygen  to 
burn  the  whole.  All  is  now  quiescent,  because  both 
materials,  being  in  the  solid  condition,  their  molecules 
are,  as  it  were,  imprisoned,  and  a  certain  degree  of  mo- 
lecular activity  is  required  to  produce  chemical  change. 
This  molecular  activity  we  can  readily  excite  by  heat, 
but  a  more  convenient,  although  less  intelligible  way, 
is  to  touch  the  mixture  with  a  drop  of  sulphuric  acid. 

Here  we  have  not  merely  a  pretty  firework,  but  an 
experiment  which  illustrates  a  very  important  phase  of 
the  phenomena  of  combustion,  and  one  of  immense 
practical  value.  I  have  chosen  this  particular  example 
because  you  are  familiar  with  both  of  the  materials 
employed.  You  have  seen  that  sugar  contains  a  large 
amount  of  combustible  carbon.  You  also  know  that 
potassic  chlorate  contains  a  large  volume  of  oxygen, 
which  can  readily  be  driven  off  by  heat ;  for  you  have 
seen  me  make  oxygen  from  this  very  salt.  You  can, 
therefore,  fully  appreciate  the  conditions  we  had  in  our 
crucible  at  the  beginning  of  the  experiment,  namely,  a 
combustible  with  the  oxygen  required  to  burn  it  in  close 
proximity.  You  will  be  prepared,  then,  to  understand — 
1.  That  the  burning  we  have  just  witnessed  does  not  dif- 
fer from  ordinary  burning,  except  in  the  single  point  I 
have  mentioned  ;  that  the  combustible  derives  its  oxygen 
from  potassic  chlorate,  instead  of  from  the  air ;  and,  2. 
that  it  is  possible  to  inclose  in  a  confined  space,  as  a 
#un -barrel  or  a  bomb,  all  the  conditions  of  combustion. 
In  a  word  this  experiment  illustrates  the  simple  theory 
of  gunpowder. 

What,  then,  is  gunpowder?     Essentially  a  mixture 


HOW   MADE.  239 

of  two  substances — saltpetre  and  charcoal,  with  merely 
a  small  amount  of  sulphur  added  to  facilitate  the  kin- 
dling of  the  charcoal.  In  the  manufacture  of  this 
explosive  agent,  as  is  well  known,  the  materials  are 
first  reduced  to  a  very  fine  powder,  and  then  inti- 
mately mixed  together.  Afterward,  by  great  pressure, 
the  mass  is  compacted  to  a  firm,  hard  cake,  which  is 
subsequently  broken  up  into  grains  of  different  sizes, 
adapted  to  various  uses.  Here  we  have  some  samples 
of  these  grains,  varying  from  the  size  of  a  walnut  to 
that  of  a  millet-seed.  These  black  grains,  although 
they  appear  so  homogeneous,  are,  in  fact,  a  very  inti- 
mate mixture  of  a  combustible  material  (charcoal  and 
a  little  sulphur)  with  a  substance  rich  in  oxygen  (salt- 
petre), and,  when  we  ignite  the  powder,  the  charcoal 
burns  at  the  expense  of  the  oxygen  of  the  saltpetre. 
Two  parallel  experiments  will  make  the  whole  matter 
clear. 

In  this  jar  we  have  about  one  gallon  (100  grains) 
of  pure  oxygen,  enough  to  combine  with  37^  grains  of 
charcoal.  This  quantity  of  charcoal  we  will  place  in  a 
copper  spoon,  and,  having  ignited  the  coal,  we  will 
plunge  it  into  the  jar  of  oxygen.  We  have  at  once  a 
brilliant  combustion,  and  a  repetition  of  the  experi- 
ment which  you  witnessed  at  the  last  lecture.  We 
then  learned  that  the  process  consists  in  the  union  of 
the  oxygen  with  the  carbon,  and  that  each  molecule  of 
oxygen  gas  actually  picks  up  an  atom  of  carbon  to  form 
a  molecule  of  carbonic  dioxide.  There  are,  therefore, 
just  as  many  molecules  in  the  jar  at  the  close  of  the  ex- 
periment as  at  the  first,  only  they  now  consist  of 
three  atoms,  instead  of  two ;  OO  has  become  OC=O. 

In  the  second  jar  is  a  cup  containing  a  small  quan- 
tity of  gunpowder,  and  so  arranged  that  the  powder 


240  GUNPOWDER. 

can  be  exploded  by  a  voltaic  battery.  As  the  oxygen- 
atoms  required  for  the  burning  are  lying  in  the  cup 
side  by  side  with  the  charcoal,  we  do  not  need  the  air 
in  our  experiment.  Accordingly,  we  have  connected 
the  jar  with  an  air-pump,  so  that  we  can  exhaust  the 
air.  .  .  .  The  gauge  of  the  pump  now  indicates  that 
the  greater  part  of  the  air  has  been  removed.  Notice 
further  that,  when  we  readmit  a  little  air,  the  mercury 
column  falls,  and  thus,  as  you  see,  this  gauge  will  tell 
us  when  any  gas  enters  the  jar.  .  .  .  Having  again 
completed  the  exhaustion,  let  us  fire  the  powder.  .  .  . 
The  powder  has  disappeared ;  but  the  gauge  indicates 
that  a  large  volume  of  gas  has  been  formed. 

A  simple  test  will  now  show  that  the  aeriform  prod- 
ucts in  the  two  last  experiments  are  identical.  Here  are 
two  glasses,  each  filled  with  lime-water.  To  one  we  will 
add  some  of  the  gas  from  the  first  jar,  pouring  it  in 
upon  the  lime-water,  and  to  the  other  we  will  add 
some  of  the  gas  from  the  gunpowder,  by  pouring  as 
before.  On  shaking  the  gas  and  liquid  together,  we 
obtain  in  both  cases  the  familiar  milky  turbid  ness 
which  indicates  the  presence  of  carbonic  dioxide.  It  is 
true  that  the  carbonic  dioxide  from  the  gunpowder  is 
not  quite  so  pure  as  that  found  in  the  other  jar,  but 
this  is  an  unessential  matter. 

Having  seen  that  gunpowder,  burnt  in  a  vacuum, 
is  quietly  resolved  into  gas,  we  will  next  take  an  equal 
amount  of  powder  and  inclose  it  in  a  pasteboard  case, 
which  we  call  a  cartridge,  using  the  same  arrangement 
for  firing  the  powder  as  before.  We  make  the  connec- 
tion, and  off  it  goes !  .  .  .  There  can  be  no  occasion, 
I  think,  to  seek  far  for  the  cause  of  the  explosion.  The 
chemical  process  must  have  been  identical  with  that  in 
our  jar  ;  but,  while  in  the  jar  there  was  room  for  all  the 


CAUSE   OF  EXPLOSIVE   FORCE.  241 

gas-molecules  formed  in  the  burning,  the  small  volume 
of  the  cartridge  could  not  hold  them,  and  they  burst  out, 
tearing  away  the  paper  walls  in  their  course.,  The  gas 
evolved  would  occupy,  at  the  ordinary  pressure  of  the 
air,  about  three  hundred  times  the  volume  of  the  powr 
der  used,  and,  if  confined  in  the  space  previously  filled 
with  the  powder,  would  exert  a  pressure  equal  to  about 
300  x  14  =  4,200  Ibs.,  or  two  tons,  on  a  square-inch.  The 
pressure  obtained  is  really  far  greater  than  this,  on  ac- 
count of  the  heat  developed  by  the  combustion.  More- 
over, as  the  powder  burns  rapidly,  this  pressure  is  sud- 
denly applied,  and  has  all  the  effect  of  an  immensely 
heavy  blow,  which  no  strength  of  materials  is  sufficient 
to  withstand.  Of  course,  any  chamber  in  which  the 
powder  is  confined  gives  way  at  the  weakest  point. 
In  the  chamber  of  a  gun  the  ball  usually  yields  before 
the  breech,  and  is  hurled  with  violence  from  the  mouth 
of  the  piece ;  but  fearful  accidents  not  unfrequently 
occur  when,  for  any  reason,  the  ball  has  been  too  tightly 
wedged,  or  when  the  metal  of  the  breech  is  too  weak. 

You  all  know  that  a  large  amount  of  gas  condensed 
into  a  small  chamber  must  exert  great  pressure,  and 
therefore  you  will  undoubtedly  regard  the  explanation 
I  have  given  of  the  force  exerted  by  gunpowder  as 
satisfactory  and  sufficient.  But,  although  this  is  the 
usual  way  of  presenting  the  phenomena,  I  am  anxi- 
ous that  you  should  view  them  in  the  light  of  our 
modern  molecular  theory,  which  gives  to  the  imagi- 
nation a  far  more  vivid  picture  of  the  manner  in  which 
the  power  acts. 

Begin  with  the  black  grains  as  they  lie  in  the  cham- 
ber of  the  gun  behind  the  ball.  You  must  remember 
that  all  the  ingredients  of  the  powder  are  in  a  solid 
condition,  and  picture  to  your  imagination  the  mole- 


242  NITRO-GLYCERINE. 

cules  as  held  in  their  places  by  those  forces  which  I 
attempted  to  make  evident  to  you  in  a  former  lecture, 
incapable  of  any  motion  except  a  slight  oscillation 
about  the  centres  of  force.  The  gun  is  now  fired,  and 
the  powder  burns.  We  need  consider  but  two  of  the 
immediate  consequences  :  first,  there  is  a  large  volume 
of  gas  formed ;  and,  secondly,  there  is  a  very  great 
amount  of  energy  developed.  Picture  to  yourselves, 
now,  an  immense  number  of  gas -molecules  suddenly 
set  free  in  the  chamber  of  the  gun,  and  animated  with 
all  the  velocity  which  great  energy  is  capable  of  im- 
parting. See  these  molecules  rushing  against  the  ball 
with  their  whole  might,  and,  when  at  last  it  starts,  im- 
parting to  the  projectile  their  moving  power,  until  it 
acquires  the  fearful  velocity  with  which  it  rushes  from 
the  mouth  of  the  gun.  The  molecules  impart  their 
motion  to  the  ball,  just  as  one  billiard-ball  imparts  mo- 
tion to  another.  The  effect  is  due  to  the  accumulation 
of  small  impulses ;  for,  although  the  power  imparted 
by  a  single  molecule  may  be  as  nothing,  the  accumu- 
lated effect  of  millions  on  millions  of  these  impulses 
becomes  immense. 

Within  a  few  years  our  community  have  become 
familiar  with  the  name  and  terrible  effects  of  a  new  ex- 
plosive agent,  called  nitro-glycerine,  and  I  feel  sure  that 
you  will  be  glad  to  be  made  acquainted  with  the  re- 
markable qualities  and  relations  of  this  truly  wonderful 
substance.  Every  one  knows  that  clear,  oily,  and  sweet- 
tasting  liquid  called  glycerine,  and  probably  most  of 
you  have  eaten  it  for  honey.  But  it  has  a  great  many 
valuable  uses,  which  may  reconcile  you  to  its  abuse  for 
adulterating  honey,  and  it  is  obtained  in  large  quanti- 
ties as  a  secondary  product  of  the  manufacture  of  soap 


PREPARATION.  243 

and  candles  from  our  common  fats.  Now,  nitro-glycer- 
ine  bears  the  same  relation  to  glycerine  that  saltpetre 
bears  to  caustic  potash.  Common  saltpetre,  which  is 
the  oxygenated  ingredient  of  gunpowder,  is  called  in 
chemistry  potassic  nitrate,  and,  although  the  com- 
mercial supply  comes  wholly  from  natural  sources, 
it  can  easily  be  made  by  the  action  of  nitric  acid  on 
caustic  potash.  My  assistant  will  pour  some  nitric  acid 
into  a  solution  of  caustic  potash,  and  you  will  soon  see 
crystals  of  saltpetre  appear,  shooting  out  from  the  sides 
of  the  dish,  whose  image  we  have  projected  on  the 
screen.  In  a  similar  way  we  can  prepare  nitro-glyce- 
rine  by  pouring  glycerine  in  a  fine  stream  into  very 
strong  nitric  acid,  rendered  more  active  by  being  mixed 
with  sulphuric  acid — oil  of  vitriol. 

We  could  easily  make  the  experiment,  but  you  could 
see  nothing.  There  is  no  apparent  change,  and  it  is  a 
remarkable  fact  that,  when  pure,  nitro-glycerine  re- 
sembles, externally,  very  closely  glycerine  itself,  and, 
like  it,  is  a  colorless,  oily  fluid — the  reddish-yellow  color 
of  the  commercial  article  being  due  to  impurities.  As 
soon  as  the  chemical  change  is  ended,  the  nitro-glycer- 
ine must  be  very  carefully  washed  with  water,  until  all 
adhering  acid  has  been  removed.  The  material  thus 
obtained  has  most  singular  qualities,  and  not  the  least 
unexpected  of  these  is  its  stability  under  ordinary  con- 
ditions. After  the  terrible  accidents  that  have  hap- 
pened, it  would,  perhaps,  be  rash  to  say  that  it  did  not 
readily  explode  ;  but  I  can  assure  you  that  it  is  not  an 
easy  matter  to  explode  pure  nitro-glycerine.  It  is  not 
nearly  so  explosive  as  gunpowder,  and  I  am  told  that 
the  flame  of  an  ordinary  match  can  be  quenched  in  it 
without  danger,  although  I  confess  that  I  should  be  un- 
willing to  try  the  experiment.  Still,  there  can  be  no 


244  NITRO-GLYCERINE. 

doubt  that,  under  ordinary  circumstances,  a  small  flame 
will  not  ignite  it.  My  knowledge  of  the  matter  is  de- 
rived from  Professor  Hill,  late  of  the  Torpedo  Station 
at  Newport,  who  has  studied  very  carefully  the  prepara- 
tion and  application  of  the  material.  He  is  of  opinion 
that  most  of  the  accidents  which  have  given  to  nitro- 
glycerine such  an  unfortunate  notoriety  have  been 
caused  by  the  use  of  an  impure  article,  and  that  proper 
care  in  its  preparation  would  greatly  lessen  the  danger 
attending  its  use.  Nitro-glycerine  is  usually  exploded, 
not  by  the  direct  application  of  heat,  but  by  a  sudden 
and  violent  concussion,  which  is  obtained  by  firing  in 
contact  with  it  a  fuse  of  some  fulminating  powder. 
The  effects  of  this  explosion  are  as  peculiar  as  the 
method  by  which  it  is  obtained,  and  I  can  best  illustrate 
the  subject  by  describing  an  experiment  with  nitro- 
glycerine which  I  witnessed  myself  at  the  Torpedo 
Station  a  few  years  since. 

It  is  so  inconvenient  to  handle  liquid  nitre-glycerine 
that  it  is  now  usual  to  mix  it  with  some  inert  and  im- 
palpable powder,  and  the  names  dualine  and  dynamite 
have  been  given  to  different  mixtures  of  this  kind ;  but 
in  both  of  these  the  powder  merely  acts  as  a  sponge. 
In  the  experiment  referred  to,  a  canister  holding  less 
than  a  pound  of  dynamite,  and  only  a  few  ounces  of 
nitro-glycerine,  was  placed  on  the  top  of  a  large  bowl- 
der-rock, weighing  two  or  three  tons.  In  order  that 
you  may  fully  appreciate  the  conditions,  I  repeat  that 
this  tin  case  was  simply  laid  on  the  top  of  the  bowlder, 
and  not  confined  in  any  way.  The  nitro-glycerine  was 
then  exploded  by  an  appropriate  fuse  fired  from  a  dis- 
tance by  electricity.  The  report  was  not  louder  than 
from  a  heavy  gun,  but  the  rock  on  which  the  canister 
lay  was  broken  into  a  thousand  fragments. 


RENDING   POWER.  245 

This  experiment  strikingly  illustrates  the  peculiar 
action  of  nitro  -  glycerine.  In  using  gunpowder  for 
blasting  it  is  necessary  to  confine  it,  by  what  is  called 
tamping,  in  the  hole  prepared  for  it  in  the  rock.  Not 
so  with  nitro-glycerine.  This,  though  it  may  be  put 
up  in  small  tin  cartridges  for  convenience,  is  placed  in 
the  drill-holes  without  tamping  of  any  kind.  Some- 
times the  liquid  itself  has  been  poured  into  the  hole, 
and  then  a  little  water  poured  on  the  top  is  the  only 
means  used  to  confine  it.  As  an  agent  for  blasting, 
nitro-glycerine  is  so  vastly  superior  to  gunpowder  that 
it  must  be  regarded  as  one  of  the  most  valuable  dis- 
coveries of  our  age.  Already  it  is  enabling  men  to 
open  tracks  for  their  iron  roads  through  mountain- 
barriers  which,  a  few  years  ago,  it  would  have  been 
thought  impracticable  to  pierce,  and,  although  its  intro- 
duction has  been  attended  with  such  terrible  accidents, 
those  best  acquainted  with  the  material  believe  that, 
with  proper  care  in  its  manufacture,  and  proper  precau- 
tions in  its  use,  it  can  be  made  as  safe  as  or  even  safer 
than  gunpowder,  and  the  Government  can  do  no  bet- 
ter service  toward  developing  the  resources  of  the  coun- 
try than  by  encouraging  such  investigations  as  have 
been  made  at  the  Torpedo  Station  at  Newport,  until  all 
the  conditions  required  for  the  safe  manufacture  and 
use  of  this  valuable  agent  are  known,  and,  when  this 
result  is  reached,  imposing  on  the  manufacturers,  deal- 
ers, and  carriers,  such  restrictions  as  the  public  safety 
requires.  Of  course,  we  cannot  expect,  thus,  to  prevent 
all  accidents.  Great  power  in  the  hands  of  ignorant 
or  careless  men  implies  great  danger.  Sleepless  vigi- 
lance is  the  condition  under  which  we  wield  all  the 
great  powers  of  modern  civilization,  and  we  cannot 


246  NITRO-GLYCERINE. 

expect  that  the  power  of  mtro-glycerine  will  be  any  ex- 
ception to  the  general  rule.1 

But,  while  nitro-glycerine  has  such  great  rending 
power,  it  has  no  value  whatever  as  a  projectile  agent. 
Exploded  in  the  chamber  of  a  gun,  it  would  burst  the 
breech  before  it  started  the  ball.  Indeed,  there  is  a 
great  popular  misapprehension  in  regard  to  the  limit 
of  the  projectile  power  of  gunpowder,  and  inventors 
are  constantly  looking  for  more  powerful  projectile 
agents  as  the  means  of  obtaining  increased  effects. 
But  a  study  of  the  mechanical  conditions  of  projec- 
tion will  show  not  only  that  gunpowder  is  most  admi- 
rably adapted  to  this  use,  but  also  that  its  capabilities 
far  exceed  the  strength  of  any  known  material,  and  the 
student  will  soon  be  convinced  that  what  is  wanted  is 
not  stronger  powder,  but  stronger  guns.  I  do  not 
mean  to  say  that  we  cannot  conceive  of  a  better  pow- 
der than  that  now  in  use,  but  merely  that  its  short- 
coming is  not  want  of  strength. 

Having  described  the  properties  of  nitro-glycerine, 
the  question  at  once  arises,  "  Can  these  singular  proper- 
ties be  explained?  "  In  order  to  answer  this  question 
I  shall  next  ask  your  attention  to  the  theory  of  its  ac- 
tion, and  I  think  you  will  find  that  our  modern  chem- 
istry is  able  to  give  a  very  intelligible  account  of  the 
phenomena  we  have  described.  I  will  begin  by  saying 
that  the  chemical  action  in  the  explosion  of  nitro-gly- 
cerine is  very  similar  to  that  in  the  burning  of  gun- 
powder. In  both  cases  we  have  the  same  two  results : 
1.  The  production  of  a  large  volume  of  gas ;  2.  The 

1  The  recent  improvements  in  the  manufacture  of  gun-cotton,  and  the 
discovery  that,  even  when  too  wet  to  burn,  it  can  be  exploded  by  con- 
cussion if  the  fuse  is  sufficiently  powerful,  promise  to  furnish  an  explo- 
sive agent  nearly  equal  to  nitro-glycerine  in  strength,  and  free  from  all 
ordinary  risks. 


MOLECULAR  STRUCTURE.  247 

liberation  of  a  large  amount  of  energy  which  gives  to 
the  confined  gas-molecules  an  immense  moving  power. 
Moreover,  essentially  the  same  aeriform  products  are 
formed  in  the  two  cases,  and  in  both  the  process  con- 
sists, for  the  most  part,  in  the  union  of  carbon  and 
hydrogen  atoms  with  oxygen.  But,  while  in  the  gun- 
powder the  carbon  and  oxygen  atoms  are  in  different 
molecules,  although  lying  side  by  side  in  the  same 
grains,  in  the  nitro-glycerine  they  are  in  different  parts 
of  the  same  molecule.  And  here  comes  our  first 
glimpse  of  the  most  recondite  chemical  principle  the 
science  has  yet  attained,  one  which  I  have  been  aiming 
to  reach  throughout  this  whole  course  of  lectures,  and 
one  which  it  will  be  my  object  in  the  three  following 
lectures  clearly  to  set  before  you.  I  can,  as  yet,  only 
state  the  principle  as  a  theorem  to  be  proved  ;  but,  if  I 
can  succeed  in  making  this  difficult  subject  clear,  I  feel 
confident  that  you  will  regard  the  proof  as  satisfactory. 
The  principle  is  this  : 

Every  molecule  has  a  definite  structure.  It  not 
only  consists  of  a  definite  kind  and  a  definite  number 
of  atoms,  but  these  atoms  are  arranged  or  grouped 
together  in  a  definite  order,  and  it  is  the  great  object 
of  modern  chemistry  to  discover  what  that  grouping 
is.  Almost  all  the  great  chemists  of  the  world  are,  at 
this  moment,  engaged  in  investigating  this  very  prob- 
lem, and,  what  is  more,  they  have  succeeded,  in  many 
cases,  in  solving  it,  and  we  have  reached  as  much  cer- 
tainty in  regard  to  the  grouping  of  the  atoms  in  the 
molecules  of  a  very  large  number  of  substances,  as  we 
have  in  regard  to  any  phenomena  so  wholly  super-sen- 
sible. For  example,  we  feel  well  assured  that  we  know 
how  the  atoms  are  grouped  in  the  molecule  of  nitro- 
glycerine, and  the  diagram  before  you  represents  in 


248  NITRO-GLYCERINE, 

H  O 

H-C-O-N  =  O  O         H    H    H          O 

H-C-0-]$r<°  N-0-0-6-C-0-N 

I  II          I     I     I  II 

H-c-o-isr=o  o       H  o  H       o 

H         O  O=N=O 

Order  of  Atoms  in  the  Molecule  Same  Order,  but  different  Form  of 

of  Nitre-glycerine.  Symbol. 

our  rude  way  the  result  we  have  reached.  The  let- 
ters signify  single  atoms,  and  the  lines  between  the 
letters  merely  show  how  the  atoms  are  severally 
united.  Begin  with  the  three  atoms  of  carbon,  which 
are  united  together,  say,  by  a  certain  force,  which  the 
lines  denote.  To  these  are  directly  united  five  atoms 
of  hydrogen,  and  then  to  each  of  the  carbon-atoms  is 
also  bound  the  atomic  group  —  O-N(^,  the  four  atoms 

of  the  group  having  a  definite  arrangement  among 
themselves.  There  is  no  virtue  in  the  mere  form  of 
the  arrangement  of  the  letters  on  the  diagram.  It  is 
perfectly  possible  that  the  atoms  may  be  arranged  so 
as  to  form  regular  geometrical  figures,  such  as  some 
theorists  have  amused  themselves  in  constructing ;  but 
we  do  not  pretend  to  have  any  accurate  knowledge  on 
this  point.  All  we  affirm  is,  that  the  atoms  are  united, 
one  with  another,  in  the  order  I  have  indicated,  and 
the  second  diagram,  in  which  the  several  atoms  are 
united  as  before,  although  the  form  of  the  arrangement 
is  different,  means,  to  the  chemist,  precisely  the  same 
thing  as  the  first. 

Now,  as  I  said,  I  present  to  you  this  diagram  of  the 
constitution  of  a  molecule  of  nitro-glycerine  simply  as 
a  theorem  to  be  proved.  As  it  hangs  before  you,  I 
have  no  doubt  that  it  will  shake  your  faith  in  the  credi- 
bility of  the  scientific  investigators  who  bring  forward 


HOW  IT  EXPLODES.  249 

this  as  the  sober  conclusion  at  which  they  have  ar- 
rived. Indeed,  when  I  first  saw  these  attempts  to 
represent  the  grouping  of  atoms,  they  appeared  to  me 
to  be  the  vagaries  of  a  diseased  scientific  imagination ; 
for,  remember,  this  molecule,  whose  structure  is  here 
portrayed,  cannot  be  larger  than  the  ^--g^-oV/roT  °f  an 
inch.  But,  as  the  evidence  pressed  upon  me,  I  re- 
luctantly examined  it.  Finding  that  it  could  not  be 
gainsaid,  I  was  forced  to  accept  the  conclusion,  and  soon 
I  found  myself  busy  at  the  same  work.  Now,  I  only 
ask  you  to  accept  this  diagram  as  a  theorem  to  be 
proved,  and,  assuming  it  for  the  time  to  represent, 
although  very  rudely,  a  real  truth,  see  how  fully  it  ex- 
plains the  properties  of  nitro-glycerme.  Indeed,  the 
facts  already  before  us  furnish  the  strongest  evidence 
possible  of  the  general  truth  of  the  principle  I  have 
asked  you  to  assume ;  for,  if  you  accept  the  principles  I 
have  previously  endeavored  to  establish,  and  once  ad- 
mit that  there  are  such  things  as  molecules  and  atoms, 
the  properties  of  nitro-glycerine  will  force  you  to  admit 
that  its  molecules  have  a  definite  structure.  See  how 
the  case  stands. 

Nitro-glycerine  has  been  analyzed,  and,  unless  the 
principles  of  our  modern  chemistry  are  all  wrong,  its 
molecules  have  the  composition  indicated  by  the  sym- 
bol C3H5N3O9.  Note  that  there  are  already  in  the  mole- 
cule nine  atoms  of  oxygen,  more  than  enough  to  satisfy 
all  the  atoms,  both  of  carbon  and  of  hydrogen.  When 
carbon  burns,  C3  only  takes  O6,  H5  only  O2i,  and  why  is 
not  the  affinity  of  these  atoms  for  oxygen  satisfied  al- 
ready ?  The  only  answer  that  can  be  suggested  is,  be- 
cause the  oxygen-atoms,  although  parts  of  the  same 
molecule,  are  not  in  combination  with  the  carbon  or 

hydrogen   atoms  in  those  molecules  ;  and  what  is  this 

18       ' 


250  NITRO-GLYCERINE. 

but  an  admission  that  the  molecules  have  a  definite 
structure  by  which  these  atoms  are  kept  apart  ? 

In  the  next  place,  admitting  that  the  structure  is 
that  represented  above,  you  see  how  the  atoms  are 
kept  apart.  Three  of  the  oxygen  -  atoms  form  the 
links,  as  it  were,  between  the  carbon  and  nitrogen 
atoms,  and  the  rest  of  the  oxygen- atoms  are  united 
with  the  nitrogen-atoms,  and  not  with  those  of  either 
carbon  or  hydrogen.  Now,  when  the  substance  ex- 
plodes, what  takes  place  is  simply  this :  The  oxygen- 
atoms  at  one  end  of  the  molecule  rush  for  the  atoms 
of  carbon  and  hydrogen  at  the  other  end,  and  the 
molecule  is  broken  up,  as  our  next  diagram  indicates ; 
only,  as  there  are  not  enough  atoms  to  form  even  mole- 

H         O 

H-C-0-N=O  H-O-H  O=0=0 

H-C-O-N(Q  H-O-H  O  =  C  =  O          ffsff 

H-C-O-N-O  H-O-H  O  =  C  =  O          NsHT 

I  I!  Water.  Carbonic  Nitrogen 

Jj  Q  Dioxide.  Gas. 

Nitro-glycerine. 

cules,  we  must  consider  that  one  atom  of  hydrogen 
and  one  of  nitrogen  are  borrowed  from  the  fragments 
of  a  neighboring  molecule,  broken  up  at  the  same 
time.  You  see,  therefore,  that  the  chemical  action  is 
very  nearly  the  same  as  in  the  burning  of  gunpowder, 
the  difference  being  that,  while  in  the  powder  the  car- 
bon and  oxygen  atoms  belong  to  different  molecules, 
in  nitro-glycerine  they  belong  to  the  same  molecule. 
In  both  cases  the  carbon  burns,  but  in  the  nitro-glycer- 
ine the  combustion  is  within  the  molecule.  This  differ- 
ence, however,  which  the  theory  indicates,  is  one  of 
great  importance,  and  shows  itself  in  the  effects  of  the 
explosion. 


EXPLANATION  OF  THE  EFFECT.  251 

In  gunpowder  the  grains  of  charcoal  and  nitre, 
although  very  small,  have  a  sensible  magnitude,  and 
consist  each  of  many  thousand  if  not  of  many  million 
molecules.  The  chemical  union  of  the  oxygen  of  the 
nitre  with  the  carbon-atoms  of  the  charcoal  can  take 
place  only  on  the  surface  of  charcoal-grains ;  the  first 
layer  of  molecules  must  be  consumed  before  the  second 
can  be  reached,  and  so  on.  Hence  the  process,  although 
very  rapid,  must  take  a  sensible  time.  In  the  nitro- 
glycerine, on  the  other  hand,  the  two  sets  of  atoms,  so 
far  from  being  in  different  grains,  are  in  one  and  the 
same  molecule,  and  the  internal  combustion  is  essen- 
tially instantaneous.  Now,  this  element  of  time  will 
explain  a  great  part  of  the  difference  in  the  effect  of 
the  two  explosions,  but  a  part  is  also  due  to  the  fact 
that  nitro  glycerine  yields  fully  nine  hundred  times  its 
volume  of  gas,  while  with  gunpowder  the  volume  is 
only  about  three  hundred  times  that  of  the  solid  grains. 
There  is  a  further  difference  in  favor  of  the  nitro-gly- 
cerine  in  the  amount  of  energy  liberated,  but  this  we 
will  leave  out  of  account,  although  it  is  worthy  of 
notice  that  energy  may  be  developed  by  internal  mo- 
lecular combustion  as  well  as  in  the  ordinary  processes 
of  burning. 

The  conditions,  then,  are  these :  "With  gunpowder 
we  have  a  volume  of  gas,  which  would  normally  occupy 
a  space  three  hundred  times  as  great  as  the  grains 
used,  liberated  rapidly,  but  still  in  a  perceptible  inter- 
val. "With  nitro-glycerine  a  volume  of  gas,  nine  hun- 
dred times  that  of  the  liquid  used,  is  set  free,  all  but 
instantaneously.  Now,  in  order  to  appreciate  the 
difference  of  effect  which  would  follow  this  difference 
of  condition,  you  must  remember  that  all  our  experi- 
ments are  made  in  air,  and  that  this  air  presses  with  an 


252  NITRO-GLYCERINE. 

enormous  weight  on  every  surface.  If  a  volume  of 
gas  is  suddenly  liberated,  it  must  lift  this  whole  weight, 
which,  therefore,  acts  as  so  much  tamping  material. 
This  weight,  moreover,  cannot  be  lifted  without  the 
expenditure  of  a  large  amount  of  work.  Let  us  make 
a  rough  estimate  of  the  amount  in  the  case  of  nitro- 
glycerine. We  will  assume  that  in  the  experiment  at 
Newport  the  quantity  exploded  yielded  a  cubic  yard 
of  gas.  Had  the  air  given  way,  instead  of  the  rock, 
the  liberation  of  this  volume  of  gas  must  have  lifted 
the  pressure  on  one  square  yard  (about  nine  tons) 
one  yard  high,  an  amount  of  work  which,  using  these 
large  units,  we  will  call  nine  yard-tons  or  about  60,000 
foot-pounds.  Moreover,  this  work  must  have  been 
done  during  the  excessively  brief  duration  of  the  explo- 
sion, and,  it  being  less  work  to  split  the  rock,  it  was 
the  rock  that  yielded,  and  not  the  atmosphere.  Com- 
pare, now,  the  case  of  gunpowder.  The  same  weight  of 
powder  would  yield  only  about  one-third  of  the  volume 
of  gas,  and  would,  therefore,  raise  the  same  weight  to 
only  one-third  of  the  height ;  doing,  therefore,  but  one- 
third  of  the  amount  of  work,  say  20,000  foot-pounds. 
Moreover,  the  duration  of  the  explosion  being  at  least 
one  hundred  times  longer  than  before,  the  work  to  be 
done  in  lifting  the  atmosphere  during  the  same  ex- 
ceedingly short  interval  would  be  only  T^  of  20,000 
foot-pounds,  or  200  foot-pounds,  and,  under  these  cir- 
cumstances, you  can  conceive  that  it  might  be  easier 
to  lift  the  air  than  to  break  the  rock. 

If  there  are  some  who  have  not  followed  me  through 
this  simple  calculation,  they  may,  perhaps,  be  able  to 
reach  clear  views  upon  the  subject  by  looking  at  the 
phenomena  in  a  somewhat  different  way.  It  can  readi- 
ly be  seen  that  the  sudden  development  of  this  large 


THE  ATMOSPHERE  AN  ANVIL.  253 

volume  of  gas,  which  becomes  at  once  a  part  of  the  at- 
mosphere, would  be  equivalent  to  a  blow  by  the  atmos- 
phere against  the  rock  ;  or,  what  would  be  a  more  ac- 
curate representation  of  the  phenomenon,  since  the  air 
is  the  larger  mass,  and  acts  as  the  anvil,  a  blow  by  the 
rock  against  the  air.  It  may  seem  very  singular  that 
our  atmosphere  can  act  as  an  anvil,  against  which  a 
rock  can  be  split,  and  yet  it  is  so,  and,  if  the  blow  has 
velocity  enough,  the  atmosphere  presents  as  effective  a 
resistance  as  would  a  granite  ledge.  The  following 
consideration  will,  I  think,  convince  you  that  this  is 
the  case  :  I  have  here  a  light  wooden  surface,  say,  one 
yard  square ;  the  pressure  of  the  air  against  the  surface 
is  equal,  as  I  just  stated,  to  about  nine  tons ;  but  the 
air  presses  equally  on  both  sides,  and  the  molecules 
have  such  great  mobility  that,  when  we  move  the  sur- 
face slowly,  they  readily  give  way,  and  we  encounter 
but  little  resistance.  If,  however,  we  push  it  rapidly 
forward,  the  resistance  greatly  increases,  for  the  air- 
molecules  must  have  time  to  change  their  position,  and 
we  encounter  them  in  their  passage.  If,  now,  we  in- 
crease the  velocity  of  the  motion  to  the  highest  speed 
ever  attained  by  a  locomotive — say,  one  and  one-fifth 
mile  per  minute — we  should  encounter  still  more  par- 
ticles, and  find  a  resistance  which  no  human  muscle 
could  overcome.  Increase  that  velocity  ten  times,  to 
twelve  miles  a  minute,  the  velocity  of  sound,  and  the 
air  would  oppose  such  a  resistance  that  our  wooden 
board  would  be  shivered  into  splinters.  Multiply  again 
the  velocity  ten  times,  and  not  even  a  plate  of  boiler- 
iron  could  withstand  the  resistance.  Multiply  the  ve- 
locity once  more  by  ten,  and  we  should  reach  the  ve- 
locity of  the  earth  in  its  orbit,  about  1,200  miles  a 
minute,  and,  to  a  body  moving  with  this  velocity,  the 


254  NITRO-GLYCEPJNE. 

comparatively  dense  air  at  the  surface  of  the  earth 
would  present  an  almost  impenetrable  barrier,  against 
which  the  firmest  rocks  might  be  broken  to  fragments. 
Indeed,  this  effect  has  been  several  times  seen,  when 
meteoric  masses,  moving  with  these  planetary  velocities, 
penetrate  our  atmosphere.  The  explosions  which  have 
been  witnessed  are  simply  the  effect  of  the  concussion 
against  the  aeriform  anvil  at  a  point  where  the  atmos- 
phere is  far  less  dense  than  it  is  here.  So,  in  the  case 
of  the  nitro-glycerine,  the  rock  strikes  the  atmosphere 
with  such  a  velocity  that  it  has  the  effect  of  a  solid  mass, 
and  the  rock  is  shivered  by  the  blow. 

In  concluding  my  illustrations  of  the  theory  of  com- 
bustion, a  few  words  in  regard  to  its  history  will  not 
be  out  of  place.  We  owe  this  theory  to  the  great 
French  chemist  Lavoisier,  who  was  murdered  by  the 
French  communists  during  the  reign  of  terror  which 
accompanied  the  first  French  Revolution.  The  theory 
came  almost  perfect  from  his  hands,  and  caused  a  revo- 
lution in  the  science  of  chemistry.  Some  would  even 
date  the  beginning  of  scientific  chemistry  at  this  epoch. 

It  is  true  that  chemistry,  as  a  science  of  exact  quan- 
titative relations,  begins  with  the  introduction  of  the 
balance  into  the  science,  and  that  Lavoisier  was  one  of 
the  first  to  recognize  the  importance  of  this  instrument 
for  investigating  chemical  problems.  But,  from  the 
beginning  of  the  seventeenth  century,  chemistry  as  a 
science  of  qualitative  relations  was  actively  studied  at 
all  the  great  centres  of  learning  in  Europe,  and  was 
illustrated  by  some  of  the  most  learned  men  of  the  age. 
For  over  a  century  previous  to  the  time  of  Lavoisier, 
who  died  in  1794,  the  doctrines  of  the  science  centred 
around  a  theory  of  combustion  which  is  known  in  his- 
tory as  the  phlogiston  theory.  This  theory  was  first  ad- 


BECHER  AND   STAHL.  255 

vanced  in  1682  by  Becher,  a  German  chemist  then  liv- 
ing in  England,  and  was  worked  out  into  a  complete 
system  some  years  later  by  Stahl.  According  to  this  the- 
ory, the  principle  of  fire  is  everywhere  diffused  through- 
out Nature,  but  enters  into  the  composition  of  different 
bodies  to  a  very  unequal  extent.  Combustible  sub- 
stances are  bodies  very  rich  in  phlogiston,  and  burning 
consists  in  the  escape  of  phlogiston  into  the  atmosphere. 
I  have  already  referred  to  this  theory,  and  shown  that 
it  was  in  variance  with  the  great  principle  of  the  law 
of  gravitation,  that  quantity  of  matter  is  proportional 
to  weight.  Still,  as  I  said  before,  this  principle  of 
Newton  made  its  way  into  chemistry  very  slowly,  and 
the  theory  of  Stahl  was  in  complete  accordance  with 
the  philosophy  of  Aristotle,  which  at  the  time  held 
an  entire  supremacy  over  the  intellectual  world.  And 
was  the  theory  wholly  false?  I  believe  not;  and  I  am 
persuaded  that  every  theory,  which  gains  among  think- 
ing men  such  universal  acceptance  as  did  this  theory 
of  Stahl,  has  its  element  of  truth.  The  men  of  the 
seventeenth  century  were  not  less  acute  thinkers  than 
ourselves,  and  we  must  be  careful  not  to  judge  of  their 
ideas  from  our  stand-point.  The  authors  of  the  theory 
never  attached  to  phlogiston  the  idea  of  weight  which 
we  necessarily  associate  with  all  matter.  It  was  to 
them  a  principle,  an  undefined  essence,  and  not  matter 
in  the  sense  we  understand  it.  Vague  and  indefinite 
idea,  no  doubt,  like  many  of  the  metaphysical  ideas  of 
the  time,  but  not  absurd.  And  that  it  was  not  absurd 
a  single  consideration  will  show.  Translate  the  word 
phlogiston  energy,  and  in  Stahl's  work  on  chemistry 
and  physics,  of  1731,  put  energy  where  he  wrote  phlo- 
giston, and  you  will  find  there  the  germs  of  our  great 
modern  doctrine  of  conservation  of  energy  —  one  of 


256  PHLOGISTON   AND   ENERGY. 

the  noblest  products  of  human  thought.  It  was  not 
a  mere  fanciful  speculation  which  ruled  the  scientific 
thought  of  Europe  for  a  century  and  a  half.  It  was  a 
really  grand  generalization  ;  but  the  generalization  was 
given  to  the  world  clothed  in  such  a  material  garb  that 
it  has  required  two  centuries  to  unwrap  the  truth. 
Still,  the  sparkle  of  the  gem  was  there,  and  men  fol- 
lowed it  until  it  led  them  into  a  clearer  day.  It  is  a 
great  error  to  suppose  that  the  theory  of  Lavoisier  su- 
perseded that  of  Stahl.  It  merely  added  to  it.  Stahl 
clearly  saw  that  the  chief  characteristic  of  burning  was 
the  development  of  energy,  and,  although  he  called 
energy  phlogiston,  and  did  not  comprehend  its  real 
essence,  he  recognized  that  it  was  a  fundamental  prin- 
ciple of  Nature.  He  did  not  understand  the  chemical 
change  which  takes  place  in  the  process,  and  this  La- 
voisier discovered.  But  both  Lavoisier  and  his  follow- 
ers, to  a  great  extent,  ignored  the  more  important  phe- 
nomenon in  magnifying  the  less,  and  it  is  only  within 
a  few  years  that  the  true  relations  of  the  two  have  been 
understood.  All  honor  to  these  great  pioneers  of  sci- 
ence, and  let  their  experience  teach  us  that,  in  science 
as  in  religion,  we  see  as  through  a  glass  darkly,  and  that 
we  must  not  attach  too  much  importance  to  the  forms 
of  thought  which,  like  all  things  human,  are  subject  to 
limitations  and  liable  to  change. 


LEOTUEE  XII. 

METATHESIS    AND    QUANTIVALENCE — ALKALIES    AND   ACIDS. 

IN  classifying  reactions  we  distinguished  besides 
analysis  and  synthesis  a  third  type  of  chemical  changes 
which  we  called  metathesis,  and  I  will  begin  my  lecture 
by  exhibiting  several  experiments  which  illustrate  pro- 
cesses of  this  kind.  This  white  solid  is  familiar  to  the 
druggists  under  the  name  of  sugar  of  lead.  It  is  made 
from  metallic  lead  and  acetic  acid,  the  acid  principle  of 
vinegar,  and  is  called  by  the  chemists  acetate  of  lead* 
It  is  a  crystalline  salt,  very  soluble  in  water,  and  this 
clear  solution  has  been  prepared  for  our  experiment. 
In  the  solution  I  now  hang  a  strip  of  thick  sheet-zinc. 
As,  however,  the  process  we  have  started  requires  sev- 
eral days,  I  have  placed  at  the  side  of  the  jar  holding  the 
solution  of  acetate  of  lead  a  similar  jar  originally  filled 
with  the  same  solution,  and  in  which  a  similar  strip  of 
zinc  was  placed  soon  after  oar  last  lecture  ;  and  notice 
that  suspended  from  the  strip  are  festoons  of  brilliant 
metallic  spangles.  These  consist  of  pure  metallic  lead, 
and  if,  after  the  process  is  ended,  we  pour  off  the  still 
clear  solution  which  has  undergone  meanwhile  no  ap- 
parent change,  we  shall  find,  on  evaporation,  that  it 


258  METATHESIS  AND   QUANTIVALENCE. 

contains  no  longer  acetate  of  lead,  but  another  white 
salt,  equally  well  known  as  acetate  of  zinc. 

This  beautiful  experiment,  known  to  the  alchemists, 
and  called  by  them  "  arbor  Saturni "  (lead-tree),  is  as 
striking  an  example  of  metathesis  as  I  can  show  you. 
Metathesis,  as  you  remember,  consists  in  an  interchange 
of  elements  between  two  substances  without  otherwise 
altering  their  structure,  and  here  there  has  been  a  simple 
interchange  between  the  two  metallic  elements,  lead  and 
zinc. 

For  a  second  experiment  we  have  prepared  a  solu- 
tion of  a  well-known  blue  salt  called  blue  vitriol,  or  sul- 
phate of  copper,  and  in  the  solution  we  will  hang  a  strip 
of  sheet-iron.  This  reaction,  like  the  other,  being  a  slow 
process,  we  were  provident  enough  to  start  the  same 
experiment  in  another  jar  in  time  to  show  you  the  re- 
sult. As  you  see,  large  spongiform  masses  of  metallic 
copper  are  suspended  to  what  remains  of  the  strip  of 
iron,  still  more  of  the  copper  sponge  has  fallen  to  the 
bottom  of  the  jar,  and  the  blue  color  has  wholly  disap- 
peared from  the  solution.  If,  now,  we  pour  off  the  so- 
lution and  evaporate  it,  we  shall  obtain  green  crystals 
of  sulphate  of  iron,  the  green  vitriol  of  commerce. 
Here,  therefore,  there  has  been  an  interchange  between 
copper  and  iron.  Let  us  now  write  these  reactions  with 
symbols : 

65.2  206.9 

(1.)  (Pb  C4  H6  04  +  Aq.)  +  Zn  =  (Zn  C4  H6  04  +  Aq.)  -f  Pb 

56  63.3 

(2.)         (Cu  S04  +  Aq.)  +  Fe  =  (Fe  SO4  +  Aq.)  +  Cu 

These  formulae  not  only  show  that  the  general  order 
of  the  two  processes  is  that  described  above,  but  they 
also  indicate  that  in  the  replacements  which  have  taken 
place  definite  proportions  by  weight  have  been  pre- 


SILVER-TREE.  259 

served.  The  symbols,  as  you  remember,  stand  for  the 
relative  weights  of  the  atoms  or  molecules  represented, 
and  the  equations  express  the  fact  that  in  the  first  re- 
action 65.2  parts  of  zinc  took  the  place  of  206.9  parts 
of  lead,  and  that  in  the  second  reaction  56  parts  of  iron 
took  the  place  of  63.3  parts  of  copper.  Now  as  is  true 
here  so,  in  general,  metathesis  consists  in  the  interchange 
of  atoms,  or  groups  of  atoms,  between  two  molecules, 
and  implies  that  the  structure  of  these  molecules  is  not 
otherwise  altered.  Such  an  interchange,  of  course,  in- 
volves the  breaking  up  of  one  set  of  molecules  and  the 
regrouping  of  the  atoms  to  form  another  set,  and  from 
this  general  point  of  view  all  reactions  are  essentially 
alike ;  but  the  cases  are  so  very  common  of  chemical 
processes  in  which  one  atom,  or  a  group  of  atoms,  is 
simply  substituted  for  another,  without  otherwise  alter- 
ing the  structure  of  the  molecules  concerned,  that  it 
is  convenient  to  study  these  reactions  by  themselves. 
Moreover,  they  have  served  to  elucidate  in  a  most  won- 
derful way  the  manner  in  which  the  atoms  and  the 
molecules  are  grouped  together.  Before,  however,  I 
attempt  to  directly  illustrate  this  point,  let  me  ask  your 
attention  to  a  few  other  examples  of  metathetical  re- 
actions in  an  order  which  will  help  to  gradually  open 
up  the  problem  of  molecular  structure. 

I  have  here  a  perfectly  colorless  solution  of  a  well- 
known  compound  of  silver  called  nitrate  of  silver,  or 
lunar  caustic.  In  this  solution  I  place  a  strip  of  metal- 
lic copper,  and  at  the  side  is  a  jar  in  which  the  same 
experiment  has  gone  on  to  completion.  Notice  that 
the  solution  has  acquired  a  decided  blue  color,  which, 
to  every  one  who  knows  that  this  is  the  characteristic 
color  of  the  salts  of  copper,  is  of  itself  a  proof  that  this 
metal  must  have  been  taken  up  from  the  copper  strip. 


260  METATHESIS   AND   QUANTIVALENCE. 

Meanwhile  a  great  abundance  of  metallic  silver  lias 
separated,  and  if  we  collect  and  weigh  the  silver,  and 
estimate  by  the  loss  the  amount  of  metallic  copper  dis- 
solved, we  shall  find  that  the  relation  of  these  weights 
is  that  of  216  to  63.3 ;  in  other  words,  two  atoms  of  sil- 
ver (weighing  each  108  m.c.)  have  been  replaced  by 
one  atom  of  copper  (weighing  63.3  m.c.).  The  reac- 
tion is  expressed  by  symbols,  thus — 

AgN03      c        c  N03      A 
Ag  N03  +        -  CllN03  +  Ag2' 

and  the  point  to  be  noticed  is  that  an  atom  of  copper  has 
taken  the  place  of  two  atoms  of  silver,  and,  in  so  doing, 
has  bound  into  one  two  previously  distinct  molecules,  so 
that  the  number  of  molecules  of  nitrate  of  copper  formed 
is  only  one  half  as  great  as  the  number  of  the  original 
molecules  of  nitrate  of  silver.  Here  we  begin  to  see 
evidence  of  molecular  structure,  for  we  have  obviously 
built  up  a  more  complex  molecule  from  two  simpler 
ones. 

In  the  three  metathetical  reactions  we  have  thus  far 
studied,  one  of  the  factors  has  always  been  a  metallic 
element.  We  will  next  pass  on  to  similar  reactions,  in 
which  both  of  the  factors  are  compound  bodies.  These 
two  "  precipitating  glasses  "  both  contain  a  solution  in 
water  of  nitrate  of  silver,  the  same  substance  which  we 
used  in  the  last  experiment.  To  the  first  we  will  now  add 
a  solution  of  common  salt — chloride  of  sodium — and  to 
the  second  a  similar  solution  of  a  less  familiar  but 
equally  definite  substance  called  chloride  of  barium. 
In  both  we  have  a  similar  result,  what  we  call  a  precipi- 
tate, and  the  white  powder  which  forms  in  clouds  and 
falls  to  the  bottom  ("  is  precipitated")  is  in  each  case 
chloride  of  silver.  The  reactions  may  be  represented 


MUTIVALENCE.  261 

in  the  following  way,  and  the  quantitative  relations  are 
exactly  those  which  the  symbols  represent : 

(1.)  (Ag  N03  +  Na  Cl  +  Aq)  =  (Na  NO3  +  Aq)  +  Ag  01. 

<2')   ( Ag  § Oa  +  Ba  Cl2  +  A(l)  =  (Ba  NOe  +  A(l )  +  2  AS  CL 

Metathetical  reactions  like  these  between  substances 
in  solution  in  water,  in  which  one  of  the  products  being 
insoluble  is  precipitated,  might  be  multiplied  almost 
indefinitely,  and  play  a  very  important  part  in  the  pro- 
cesses of  chemical  analysis.  The  two  we  have  chosen 
as  illustrations  were  selected,  in  the  first  place,  on  ac- 
count of  their  simplicity,  and,  in  the  second  place,  be- 
cause they  make  prominent  a  point  to  which  I  have 
already  referred,  and  which  I  wish  still  further  to  press 
upon  your  notice. 

It  will  be  seen  that,  while  one  atom  of  sodium  (Na) 
replaces  one  atom  of  silver  (Ag),  one  atom  of  barium 
(Ba)  replaces  two  atoms  of  silver  (Ag2).  Hence,  while  in 
a  certain  sense  the  atom  of  sodium  may  be  said  to  be  the 
equivalent  of  one  atom  of  silver,  the  atom  of  barium  is 
the  equivalent  of  two  atoms  of  silver ;  and  there  are 
also  elementary  substances  whose  atoms  are  the  equiva- 
lents of  three  atoms  of  silver,  others  whose  atoms  are 
the  equivalents  of  four  atoms  of  silver,  and  others 
whose  atoms  are  the  equivalents  of  five  and  even  of 
six  atoms  of  silver.  This  relation  of  the  chemical  atoms 
is  what  we  call  their  quantivalence,  and  we  distinguish 
atoms  as  univalent,  bivalent,  trivalent,  quadrivalent, 
quinquivalent,  or  sexivalent,  according  as  they  replace, 
and  in  that  sense  are  the  equivalents  of  one,  two,  three, 
four,  five,  or  six  atoms  of  silver. 

It  need  hardly  be  added  that  any  other  univalent 
atom  like  the  atom  of  hydrogen  or  the  atom  of  sodium 


262 


METATHESIS  AND   QUANTIVALENCE. 


might  be  taken  as  our  standard  of  comparison,  as  well 
as  the  atom  of  silver,  and  that  the  quantivalence  of  an 
atom  may  be  measured  not  only  by  univalent  atoms, 
but  also  equally  well  by  atoms  of  higher  quantivalence. 
Thus  a  sexivalent  atom  will  replace  two  trivalent  or 
three  bivalent  atoms,  as  well  as  six  of  the  unit  value. 

It  would  not  be  difficult  to  find  metathetical  reactions 
which  illustrate  the  higher  degrees  of  quantivalence; 
but  such  reactions  are  far  less  simple  than  those  we 
have  shown,  and  to  the  beginner  in  the  study  of  chem- 
istry the  point  to  be  illustrated  is  in  most  cases  obscured 
by  confusing  circumstances.  For  this  reason,  having 
seen  the  simplest  illustrations  of  a  difference  of  replac- 
ing power,  we  shall  understand  this  important  doctrine 
of  quantivalence  better  if  we  now  approach  the  subject 
from  a  different  direction. 

The  quantivalence  of  an  atom  is  shown  not  only  by 
its  capacity  of  replacing  other  atoms,  but  also  by  its 
power  of  uniting  with  other  atoms,  by  what  has  been 
called  its  atom-fixing  power. 


HC1 

H20 

H3N 

H40 

ISTaCl 

HgCl2 

SbCl3 

CC14 

PCI, 

CrF. 

H20 

HgO 

NOC1 

CO2 

POCls 

Cr03 

COC12 

Cr02Cl2 

COH2 

The  diagram  on  the  curtain  before  us  illustrates  the 
truth  we  have  to  present.  The  story,  indeed,  is  here 
told  in  our  chemical  hieroglyphics,  but  let  us  try  to  de- 
cipher them.  In  attacking  our  work,  let  us  not  fail  to 
remember  that  these  symbols  really  exhibit  the  con- 
stitution of  the  molecules  of  the  definite  substances 


THE  BASIS  OF  FACT.  263 

they  represent.  The  symbol  H2O,  for  example,  shows 
that  a  molecule  of  water  consists  of  two  atoms  of 
hydrogen  and  one  of  oxygen.  Kemember  that  this 
symbol  is  not  the  expression  of  a  mere  hypothesis, 
but  represents  the  results  of  actual  experiment.  In 
a  former  lecture  we  have  dwelt  at  length  on  the  evi- 
dence on  which  it  is  based.  We  cannot  continually 
retrace  our  steps  ;  but  be  sure  that  you  recall  this  evi- 
dence, so  that  we  may  plant  the  ladder,  on  which  we 
shall  attempt  to  climb  higher,  on  firm  ground.  Now, 
what  is  true  of  the  symbol  of  water,  is  true  of  all  the 
symbols  on  this  diagram.  There  is  not  one  of  them 
in  regard  to  which  there  is  a  shade  of  doubt.  Our 
atoms  may  be  mere  fancies,  I  admit,  but,  like  the  mag- 
nitudes we  call  waves  of  light,  the  magnitudes  we  have 
measured  and  called  atoms  must  be  magnitudes  of 
something,  however  greatly  our  conceptions  in  regard 
to  that  something  may  change.  Our  whole  atomic 
theory  may  pass,  the  words  molecule  and  atom  may  be 
forgotten ;  but  it  will  never  cease  to  be  true  that  the 
magnitude  which  w^e  now  call  a  molecule  of  water  con- 
sists of  two  of  the  magnitudes  which,  in  the  year  1884, 
were  called  atoms  of  hydrogen,  and  of  one  of  the  mag- 
nitudes which  were  called,  at  the  same  period,  atoms 
of  oxygen. 

Look,  now,  at  the  first  line  of  symbols,  and  see  in 
what  a  remarkable  relation  the  atoms  there  repre- 
sented stand  to  each  other.  In  a  molecule  of  hydro- 
chloric-acid gas  (HC1),  one  atom  of  chlorine  is  united  to 
one  atom  of  hydrogen.  In  the  molecule  of  water  (H2O) 
one  atom  of  oxygen  is  united  to  two  of  hydrogen.  In 
the  molecule  of  ammonia  gas  (NH3)  one  atom  of  nitro- 
gen is  united  to  three  atoms  of  hydrogen,  and  in  the 
molecule  of  marsh  gas  (CH4)  the  atom  of  carbon  is 


264  QUANTIVALENCE   AND   METATHESIS. 

united  to  four  atoms  of  hydrogen.  It  would  appear, 
then,  that  the  atoms  of  chlorine,  oxygen,  nitrogen,  and 
carbon,  have  different  powers  of  combination,  uniting 
respectively  with  one,  two,  three,  and  four  atoms  of 
hydrogen.  In  order  to  assure  yourselves  that  this  rela- 
tion is  not  an  illusion,  depending  on  the  collocation  of 
selected  symbols,  but  results  from  a  definite  quality  of 
the  several  atoms,  examine  the  symbols  of  the  second 
line,  and  you  wrill  see  that,  in  a  similar  way,  the  atoms 
of  sodium  (Na),  mercury  (Hg),  antimony  (Sb),  carbon  (C), 
and  phosphorus  (P),  unite  respectively  with  one,  two, 
three,  four,  and  five  atoms  of  chlorine.  Moreover,  on 
comparing  the  two  lines,  notice  that  the  atom  of  chlo- 
rine, which  combines  with  one  atom  of  hydrogen,  com- 
bines also  with  one  atom  of  sodium.  Again  notice 
that  the  atom  of  carbon,  which  combines  with  four 
atoms  of  hydrogen,  combines  also  with  four  atoms  of 
chlorine.  Further,  observe  on  the  third  line  that  the 
atom  of  mercury,  which  combines  with  two  atoms  of 
chlorine,  combines  with  only  one  of  oxygen  ;  and  that 
the  atom  of  carbon,  which  combines  with  either  four 
atoms  of  chlorine  or  four  atoms  of  hydrogen,  combines 
with  two  atoms  of  oxygen ;  and  compare  with  these 
facts  those  first  noticed,  that  the  atom  of  oxygen  com- 
bines with  two  atoms  of  hydrogen,  and  the  atom  of 
chlorine  with  but  one. 

Eelations  so  far-reaching  and  so  intricate  as  these 
cannot  be  accidental ;  and  when  you  are  told  that  the 
examples  here  given  have  been  selected,  on  account  of 
their  simplicity,  from  a  countless  number  of  instances 
in  which  similar  relations  have  been  observed,  you  will 
not  be  satisfied  until  you  find  some  explanation  of  the 
cause  of  these  facts. 

The  explanation  which  our  modern  chemistry  gives 


ATOMIC  BONDS.  265 

is  this :  It  is  assumed  that  each  of  the  elementary  atoms 
has  a  certain  definite  number  of  bonds,  and  that  by  these 
alone  it  can  be  united  to  other  atoms.  If  you  wish  to 
clothe  this  abstract  idea  in  a  material  conception,  picture 
these  bonds  as  so  many  hooks,  or,  what  is  probably  nearer 
the  truth,  regard  them  as  poles  like  those  of  a  magneto 
If  we  have  grasped  this  idea,  let  us  turn  back  to  our  dia- 
gram and  we  shall  find  that  the  relations  we  had  but 
dimly  seen  have  become  clear  and  intelligible.  The 
hydrogen,  sodium  and  chlorine  atoms  have  only  one  bond 
or  pole,  and  hence,  in  combining  with  each  other,  they 
can  only  unite  in  pairs.  The  oxygen-atom  has  two 
bonds  or  poles,  and  can  combine,  therefore,  with  two 
hydrogen-atoms,  one  at  each  pole.  The  mercury-atom 
has  also  two  bonds,  and  takes,  in  a  similar  manner,  two 
atoms  of  chlorine ;  but  it  can  only  combine  with  a  sin- 
gle atom  of  oxygen,  for  the  two  poles  of  one  just  satisfy 
the  two  poles  of  the  other.  Again,  the  atom  of  car- 
bon has  four  bonds,  which  may  be  satisfied  by  either 
four  atoms  of  hydrogen,  or  four  atoms  of  chlorine,  or 
two  atoms  of  oxygen,  or  one  atom  of  oxygen  and  two 
of  chlorine,  or,  lastly,  one  atom  of  oxygen  and  two  of 
hydrogen.  Further,  the  atom  of  phosphorus  has  five 
bonds,  and  holds  five  atoms  of  chlorine,  or  three  atoms 
of  chlorine  and  one  of  oxygen.  Finally,  the  chromium 
atom  binds  six  atoms  of  fluorine,  or  three  of  oxygen,  or 
two  of  oxygen  and  two  of  chlorine.  This  quality  of  the 
atoms,  which  we  endeavor  to  represent  to  our  minds  by 
the  conception  of  hooks,  bonds,  or  poles,  is  precisely 
the  same  quality  which  determines  its  power  of  replace- 
ment, and  we  use  the  terms  univalent,  bivalent,  trivalent, 
quadrivalent,  quinquivalent,  sexivalent,  etc.,  to  designate 
the  atoms  which  have  one,  two,  three,  four,  five,  six, 
etc.,  hooks,  bonds,  or  poles,  respectively. 

19 


1 

i 

-N- 

-0- 

i 

i 

i 

-P- 

-Si- 

1 

i 

1 

-Sb- 

-Sn- 

i 

i 

i 

-As- 

-Ti- 

i 

i 

i 

-B- 

-Pt- 

i 

i 

i 

-Au- 

-Zr- 

266  QUANTIVALENCE  AND   METATHESIS. 

H-  -O- 

Cl-  -S- 

F-  -Ca- 

Z-  -Mg- 

Na-  -Hg- 

Ag-         -Zn- 

\ 

In  the  above  diagram  we  have  classified  a  few  only 
of  the  more  important  elementary  atoms  according  to 
their  quantivalence,  and  the  diagram  also  shows  how, 
by  a  slight  addition  to  our  symbolical  notation,  we  can 
indicate  the  number  of  bonds  in  each  case.  In  writing 
symbols  of  molecules,  a  dash  between  two  letters  indi- 
cates the  union  of  two  bonds,  and  one  bond  or  pole  on 
each  atom  is  then  said  to  be  closed.  Two  dashes  indi- 
cate that  two  bonds  on  each  atom  are  closed — and  so 
with  a  larger  number.  The  next  diagram  is  in  part 
a  repetition  of  that  on  page  262,  with  the  exception 

that  the  bonds  are  indicated. 

H  H 

H-H  H-O-H  H-N-H  H-C-H 

Cl  Cl 

H-C1          Cl-Hg-Cl         Cl-Sb-Cl        C1-C-C1 

Cl 
Hg=0  C1-N=0          0  =  C  =  0 

You  notice  that  this  idea  of  quantivalence  suggests, 
or,  rather,  as  I  should  say,  implies  the  idea  that  the 
molecules  have  a  definite  structure.  Thus  in  the  mole- 


QUANTIVALENCE   IMPLIES  STRUCTURE.  267 

cule  CH4  we  conceive  that  the  carbon-atom  is  united  at 
four  distinct  points  with  the  four  hydrogen-atoms. 
There  is  not  an  indiscriminate  grouping  of  the  five 
atoms,  but  a  definite  arrangement  with  the  carbon- 
atom  at  the  centre  of  the  system.  So,  also,  in  CC14, 
which  has  the  same  structure  as  CH4,  determined,  as 
before,  by  the  quadrivalence  of  the  nucleus.  Passing 
next  to  CO2  we  find  an  equally  definite  structure,  the 
four  bonds  of  the  same  nucleus  being  satisfied  by  two 
bivalent  atoms  of  oxygen ;  and  intermediate  in  struct- 
ure, between  the  two  molecules  last  mentioned,  w^e  have 
the  molecule  of  phosgene  gas,  COC12,  and  the  molecule 
of  formic  aldehyde,  COH2. 

The  symbols  of  these  molecules  indicate  an  obvious 
limitation  to  this  idea  of  structure,  which  must  not  be 
overlooked,  and  which  cannot  too  early  be  called  to 
your  notice.  All  that  we,  as  yet,  feel  justified  in  infer- 
ring from  the  phenomena  we  have  described,  are  simply 
the  facts  that  in  the  molecule  CC14,  for  example,  the 
four  chlorine-atoms  are  united  to  the  carbon-nucleus 
by  four  different  bonds,  and  that  in  the  molecule  CO2 
the  two  oxygen-atoms  are  united  to  the  same  nucleus, 
each  by  two  bonds.  Further  than  this  we  assert  noth- 
ing. It  may  hereafter  appear  that  the  different  bonds 
of  the  carbon-atom  have  different  values ;  or,  perhaps, 
have  a  fixed  position,  and  that  there  are  distinctions  of 
right  and  left,  top  and  bottom,  or  the  like  ;  but,  until  we 
are  acquainted  with  phenomena  which  require  assump- 
tions of  this  sort,  we  may  group  our  symbols  around  the 
nucleus  of  the  molecule  as  we  find  most  convenient, 
provided  only  we  satisfy  the  condition  of  quantivalence. 
Thus  it  is  unimportant  whether  we  write 

Cl 

Cl-Hg-Cl,      or      Hg(g|;       C1-C-C1,     or      O-O 

O  01. 


268  QUANTIVALENCE  AND   METATHESIS. 

The  quantivalence  of  the  atoms,  moreover,  is  by  no 
means  an  invariable  quality  ;  but  this  circumstance 
does  not  in  the  least  obscure  the  general  principle  we 
have  been  discussing  :  because,  in  the  first  place,  any 
change  in  the  quantivalence  of  an  atom  is  accompanied 
with  a  change  in  all  its  chemical  relations  ;  and,  in  the 
second  place,  the  change  is  circumscribed  by  definite 
limits,  which  are  easily  defined.  This  point  will  be 
best  illustrated  by  a  few  examples. 

When  in  a  previous  lecture,  as  an  example  of  a 
synthetical  process,  we  united  ammonia  gas  with  hydro- 
chloric acid,  there  was  a  change  in  the  quantivalence 
of  the  nitrogen-atom,  from  three  to  five,  as  w^ill  be  seen 
on  comparing  the  symbol  of  the  first  factor  with  the 
sole  product  of  the  reaction  : 

H  H 

I  TT  I 

H-N  \N-C1 

i  H     ' 

H  H 

Ammonia  Gas.  Ammonic  Chloride. 

Now,  from  ammonia  gas  can  be  derived  a  large  class 
of  compounds,  in  all  of  which  nitrogen  is  trivalent  ;  and, 
in  like  manner,  from  ammonic  chloride  can  be  derived 
another  class  of  compounds,  in  which  nitrogen  is  quin- 
quivalent ;  but,  although  they  all  contain  the  same  atom 
as  a  nucleus,  the  two  classes  differ  from  each  other  as 
widely  as  if  they  were  composed  of  different  elements. 
A  similar  fact  is  true  of  phosphorus,  which  forms  two 
well-marked  chlorides  : 

01  Cl 


01-P  )P-C1 


1  Cl 

Cl  Cl 

Phosphorous  Chloride.  Phosphoric  Chloride. 

One  of  the  most  striking  instances  of  the  variation 
of  quantivalence  is  to  be  found  in  the  atom  of  man- 


VARIATIONS   OF   QUANTI VALENCE.  269 

ganese.  This  elementary  substance  forms  no  less  than 
four  compounds  with  fluorine,  whose  molecules  have 
probably  the  constitution  represented  by  the  symbols 

given  below : 

F  F      F 

F-Ma-F  F-Hn-F  F-Mn-Mn-F 

i  i        i 

F  F      F 

F    F 

\  / 

F-Mn-F 

/  \ 

F    F 

In  the  first,  the  manganese-atom  is  bivalent ;  in  the 
second  and  third  it  is  quadrivalent ;  and  in  the  last, 
sexivalent.  The  third  molecule,  it  will  be  noticed, 
contains  two  quadrivalent  atoms  of  manganese,  united 
by  a  single  bond,  and  the  two  together  form  a  complex 
nucleus,  which  is  sexivalent.  Here,  as  in  the  previous 
examples,  it  is  true  that  there  is  a  distinct  class  of  com- 
pounds corresponding  to  each  of  the  four  conditions  of 
the  nucleus,  and  that  the  difference  between  the  chem- 
ical relations  of  the  bivalent  and  those  of  the  sexiva- 
lent atom  of  manganese  is  almost  as  great  as  that  be- 
tween the  atom  of  zinc  and  the  atom  of  sulphur. 

The  compounds  of  iron  furnish  a  more  familiar  ex- 
ample of  the  effect  produced  by  a  variation  of  quanti va- 
lence, than  either  of  those  which  have  been  adduced. 
There  are  two  classes  of  these  compounds,  which  are 
distinguished  in  chemistry  as  the  ferrous  and  the  fer- 
ric compounds.  The  first  class  consists  of  molecules, 
of  which  the  nucleus  is  a  bivalent  atom  of  iron,  while 
the  molecules  of  the  second  class  are  grouped  around  a 
nucleus,  consisting  of  two  quadrivalent  atoms  united  as 
explained  above.  Thus  the  symbols  of  ferrous  and 
ferric  chloride  are : 


270  QUANTIVALENCE   AND   METATHESIS. 

Cl      01 

FeCl2    or    Cl-Fe-Cl,     and    FeaCl6    or    Cl-Fe-Fe-01. 

i         i 
Cl     CL 

Now,  I  have  before  me  four  glasses,  which  contain 
solutions  in  water  of 


FeCl2,  Fe2Cl6,  CuCl2 

Ferrous  Chloride,        Ferric  Chloride,        Cupric  Chloride,     and    Nickel  Chloride  ; 

and  I  will  add  to  each  glass  a  portion  of  a  solution  of 
a  yellow  salt,  which  is  well  known  in  commerce,  under 
the  name  of  yellow  prussiate  of  potash,  and  in  chemis- 
try as  potassic  ferrocyanide.  Notice,  in  the  first  place, 
what  a  different  effect  the  reagent  produces  on  the  last 
two  solutions.  From  the  solution  of  cupric  chloride, 
we  obtain  a  red  precipitate,  and,  from  the  solution 
of  nickel  chloride,  a  white  precipitate.  Next,  we  will 
add  the  same  reagent  to  the  solutions  of  the  two  com- 
pounds of  iron,  and,  as  you  see,  the  difference  of  effect 
produced  is  even  greater  than  before.  Moreover,  if, 
going  behind  the  outward  manifestations,  you  study 
the  constitution  of  the  products  formed,  you  will  find 
that  the  variations  of  color  correspond  to  more  funda- 
mental differences  in  the  case  of  the  two  conditions 
of  iron  than  in  that  of  the  two  separate  elements,  cop- 
per and  nickel.  The  result,  then,  at  which  we  arrive, 
is  this,  that,  although  a  fixed  quantivalence  is  not  an 
invariable  of  quality  of  every  atom,  it  is  at  least  an  in- 
variable quality  of  each  condition  of  every  given  atom, 
and  that,  in  every  marked  class  of  compounds  of  any 
elementary  substance,  the  atoms  of  that  element  always 
have  the  same  quantivalence. 

Lastly,  as  to  the  limits  to  which  this  variation  of 
quantivalence  may  extend.  There  are  several  of  the 
chemical  elements,  and  these  among  the  most  impor- 


LAW  OF  THE  VARIATION.  271 

tant  and  most  widely  distributed,  whose  quantivalence 
appears  to  be  invariable.  This  is  especially  true  of 
hydrogen,  it  is  likewise  true  of  the  alkaline  metals,  lith- 
ium, sodium,  potassium,  caesium,  and  rubidium,  and  it 
is  also  true  of  silver,  all  elements  whose  atoms  are  univa- 
lent.  It  is  further  true  of  the  trivalent  element  boron. 
Again,  oxygen  is  always  bivalent,  and  so  are  also  the 
metallic  radicals  of  the  alkaline  earths,  calcium,  barium, 
strontium,  and  magnesium,  and  so  are,  moreover,  the 
well-known  metallic  elements,  lead,  zinc,  and  cadmium. 
Lastly,  aluminum,  titanium,  silicon,  and  carbon,  are  al- 
ways quadrivalent,  although,  in  the  single  instance  of  the 
molecule,  CO,  the  carbon-atom  appears  to  be  bivalent. 
But,  in  addition  to  the  fact  that  the  variations  in 
quantivalence  are  confined  to  a  limited  number  of  the 
elementary  atoms,  these  variations  appear  to  follow  a 
remarkable  law,  which  is  thought  to  point  to  an  ex- 
planation of  their  cause.  As  is  shown  in  this  diagram, 
the  successive  degrees  of  quantivalence  in  gold  and 
phosphorus  follow  the  order  of  the  odd  number : 

AuCl  AuCl3 

PCls  PC16 

while  those  of  manganese  follow  the  order  of  the  even 
numbers : 

MnF2  MnF4  MnF6 

Now,  what  is  true  of  these  atoms  is,  in  general,  true 
of  the  atoms  of  all  those  elements  which  have  several 
degrees  of  quantivalence :  at  each  successive  step  the 
quantivalence  increases  by  two  bonds,  and  never  by  a 
single  bond.  The  explanation  of  the  fact  is  thought  to 
be  that  the  bonds  of  any  atom,  when  not  in  use  to  hold 
other  atoms,  are  satisfied  by  each  other,  and  that,  so  far 
as  these  unused  bonds  are  concerned,  the  atom  is  in 


272  QUANTIVALENCE   AND  METATHESIS. 

the  condition  of  a  horseshoe  magnet,  with  its  north  pole 
directed  toward  and  neutralized  by  its  south  pole.  Thus 
it  is  assumed  that,  in  both  of  the  two  compounds  of  car- 
bon and  oxygen,  the  carbon  atom  is  quadrivalent,  the 
only  difference  being  that,  while  in  CO2  all  four  bonds 
are  employed  to  hold  the  two  atoms  of  oxygen,  in  CO 
only  two  are  so  used,  the  other  two  neutralizing  each 
other  thus  : 

OCO  CC=O. 

Of  course,  then,  if  the  unused  bonds  are  in  all  cases 
neutralized  in  this  way,  it  must  be  that  the  quantiva- 
lence  of  an  atom  will  fall  off  from  the  highest  degree 
of  which  it  it  susceptible,  by  two  bonds  at  each  step ; 
so  that,  if  the  highest  degree  is  odd,  all  must  be  odd, 
and,  if  the  highest  is  even,  all  must  be  even,  as  in  the 
illustrations  given  above.  Atoms  with  odd  degrees  of 
quantivalence  have  been  called  perissads,  and  those  with 
even  degrees  have  been  called  artiads,  and  the  classifi- 
cation appears  to  be  a  fundamental  one  ;  but  there  are 
important  exceptions  to  the  general  principle,  which 
have  never  yet  been  reconciled  with  the  theory, 

The  doctrine  of  quantivalence,  which  we  have  en- 
deavored to  illustrate  in  this  lecture,  is  one  of  the  dis- 
tinctive features  in  which  the  new  chemistry  differs 
from  the  old,  and  the  recognition  of  the  fact  that  a  defi- 
nite quantivalence  is  an  inherent  quality  of  each  ele- 
mentary atom  was  one  of  the  chief  causes  of  the  revo- 
lution in  the  science  which  has  recently  taken  place. 
In  the  old  chemistry,  the  question  of  how  the  element- 
ary substances  were  united  in  a  compound  was  hardly 
raised,  much  less  answered ;  but  now  the  manner  in 
which  the  atoms  are  grouped  together  in  the  molecule 
has  become  an  all-important  question.  Every  mole- 
cule is  a  unit  in  which  all  the  atoms  are  joined  to- 


ATOMIC  CLAMPS.  273 

gether  by  their  several  bonds,  and  it  becomes  an  object 
of  investigation  to  determine  the  exact  manner  in  which 
the  molecular  structure  is  built  up.  Moreover,  it  ap- 
pears that  the  qualities  and  chemical  relations  of  a  com- 
pound are  determined  fully  as  much  by  the  structure 
of  its  molecules  as  by  the  nature  of  the  atoms  of  which 
the  molecules  consist.  For  example,  it  was  formerly 
supposed  that  the  qualities  of  an  alkali  or  an  acid  were 
simply  the  characteristics  of  the  compounds  of  certain 
elements  with  oxygen,  but  it  now  appears  that  they  are 
the  result  of  a  definite  molecular  structure,  and  are 
only  slightly  modified  by  the  characteristics  of  the  in- 
dividual atoms  which  may  chance  to  be  the  nucleus  of 
the  molecule. 

We  are  thus  fairly  brought  face  to  face  with  the 
question  of  molecular  structure  that  is  to  occupy  our 
attention  during  the  remainder  of  this  course  of  lect- 
ures. In  regard  to  this  question,  there  are  a  few  pre- 
liminary points  which  need  barely  be  mentioned,  as 
they  can  easily  be  apprehended,  and  require,  therefore, 
no  extended  illustration.  It  is  evident  that  with  univa- 
lent  atoms  solely  we  can  only  form  molecules  con- 
sisting of  two  atoms,  like  Na-Cl,  or  H-Br.  When  we 
introduce  bivalent  atoms  the  structure  becomes  more 
complex— as  in  H-O-H  or  K-O-C1.  With  several  biva- 
lent atoms  we  can  form  molecules  in  which  the  atoms 
seem  to  be  strung  together  in  a  chain,  sometimes  of 
great  extent,  as — 

H~°7Ca-0-H,     or    H-0-Pb-O-Pb-O-Pb-O-H. 

Calcic  Hydrate.  Triplumbic  Hydrate. 

And,  with  atoms  of  higher  quanti valence,  we  obtain 
groups  of  very  great  complexity,  of  which  the  multiva- 
lent  atom  l  is  the  nucleus,  and  serves  to  bind  together 

1  The  atom  with  a  high  degree  of  quantivalence. 


274  QUANTIVALENCE  AND   METATHESIS. 

the  parts  of  the  molecule.  The  molecule  of  calcic  sul- 
phate, for  example,  is  supposed  to  have  the  complex  con- 

/0N    ^O 

Ca          S 

^O/    ^O 

Calcic  Sulphate. 

stitution  which  our  symbol  indicates,  and  it  will  be  seen 
that  it  is  the  sexivalent  atom  of  sulphur,  which  is  the 
nucleus  of  the  group,  and  holds  the  atoms  together.  So, 
also,  in  the  still  more  complex  molecule  of  alum,  the 
double  atom  of  aluminum  is  the  nucleus  of  the  group, 

O    O 

V 

O  O    O  O 

K-0-S-O-A1-A1-O-S-0-K 

ii  ii  ii 

O  00  O 

\    / 

S 
O    O 

Potassic-Aluminic  Sulphate  (Alum). 

and  unites  the  several  parts,  while  the  four  sexivalent 
atoms  of  sulphur  are  the  centres  of  subordinate  groups 
connected  with  this  nucleus.  Notice  that  all  the  atoms 
are  united  by  their  respective  bonds,  and  that  to  each 
set  is  assigned  a  definite  quanti valence,  and  you  can 
hardly  fail  to  appreciate  the  important  fundamental 
principles  of  our  modern  chemistry,  which  I  have  been 
endeavoring  to  illustrate.  They  may  be  summed  up  in 
the  following  terms : 

The  integrity  of  every  complex  molecule  depends  on 
the  multivalence  of  one  or  more  of  its  atoms,  and  no 
such  molecule  can  exist  unless  its  parts  are  bound  to- 
gether by  these  atomic  clamps. 

Such  symbols  as  those  just  given,  by  which  we  at- 
tempt to  indicate  the  relations  of  the  parts  of  a  mole- 


GRAPHIC  SYMBOLS.  275 

cule,  are  called  graphic  or  sometimes  rational  symbols, 
and  are  to  be  distinguished  from  those  we  have  hitherto 
used,  which,  as  they  represent  simply  the  results  of  ex- 
periment, are  known  as  empirical  symbols.  Of  course, 
these  graphic  symbols  are  the  expressions  of  our  theo- 
retical conceptions,  and  must  survive  or  perish  with  the 
theory  that  gave  them  birth.  But,  absurd  as  these  con- 
ceptions certainly  would  be  if  we  supposed  them  realized 
in  the  concrete  forms  which  our  diagrams  embody,  yet, 
when  regarded  as  aids  to  the  attainment  of  general 
truths,  which  in  their  essence  are  still  incomprehensi- 
ble, these  crude  and  mechanical  ideals  have  the  greatest 
value,  and  become  very  important  aids  to  the  study  of 
chemical  science. 

The  molecular  structure  of  bodies  is  inferred  chiefly 
from  the  reactions  of  which  they  are  susceptible,  or  by 
which  they  are  formed,  and  I  now  propose  to  ask  you 
to  study  with  me  a  number  of  chemical  processes  which 
I  have  selected  with  a  view  of  illustrating  the  structure 
of  a  few  of  the  more  important  classes  of  chemical 
compounds.  The  processes  best  adapted  for  our  pur- 
pose, and  therefore  selected,  are  chiefly  examples  of 
metathesis,  and  incidentally  we  shall  become  acquainted 
with  a  still  larger  number  of  this  class  of  chemical  re- 
actions, and  from  general  considerations  it  can  easily  be 
seen  in  what  way  metathetical  reactions  exhibit  the 
structure  of  molecules. 

Metathesis  consists,  as  we  have  seen,  in  the  inter- 
change of  atoms  between  two  molecules,  and  implies 
that  the  preexisting  relations  of  the  atoms  in  the  mole- 
cules are  not  otherwise  altered.  If,  then,  by  clamping 
together  two  simpler  molecules  by  means  of  a  multiva- 
lent  atom  (as  in  the  reaction  on  page  260),  we  bind  them 
into  a  more  complex  whole,  it  is  evident  that  we  can  in- 


276  QUANTIVALENCE   AND   METATHESIS. 

fer  the  structure  of  the  whole  from  that  of  the  parts, 
and  in  this  way  reason  up  from  the  simplest  to  the  most 
complex  compounds. 

In  a  similar  way,  by  substituting  univalent  for  mul- 
tivalent  atoms,  we  can  often  reverse  this  constructive 
process,  and  infer  the  structure  of  a  molecule  from  the 
manner  in  which  it  breaks  up.  In  a  word,  we  know 
how  the  building  is  constructed,  because  we  either  built 
it  or  tore  it  down. 

In  this  connection  the  first  reaction  which  I  shall 
bring  to  your  notice  is  that  of  metallic  sodium  on  water, 
with  the  view  of  exhibiting  the  structure  of  two  very 
important  and  characteristic  classes  of  compounds  long 
known  in  the  arts  as  well  as  in  science,  under  the  names 
of  alkalies  and  acids. 

The  effect  of  pure  sodium  on  water  is  so  violent  that 
we  find  it  convenient  to  moderate  the  action  by  amalga- 
mating the  metal  with  mercury,  which,  without  in  the 
least  degree  altering  the  relations  of  the  sodium  to  the 
water,  reduces  the  rapidity  of  the  chemical  process.  We 
will,  now,  pass  under  this  glass  bell,  which  is  filled  with 
water,  and  standing  on  the  shelf  of  the  pneumatic  trough, 
a  bit  of  this  sodium  amalgam.  You  notice  a  rapid  evo- 
lution of  gas,  which  soon  nearly  fills  the  bell.  Let  us 
examine  this  gas.  On  bringing  the  open  mouth  of  the 
bell  near  a  candle-flame,  the  gas  takes  fire  and  burns 
with  the  familiar  appearance  of  hydrogen,  and  this  is 
sufficient  to  assure  you  that  the  product  with  which  we 
are  here  dealing  is  hydrogen  gas.  But  what  is  the  other 
product  of  the  reaction  ?  To  discover  this,  we  will  next 
place  another  lump,  this  time  of  the  pure  metal,  on  an 
open  pan  of  water.  The  sodium  being  lighter  than  the 
water,  floats  on  the  surface,  and  ,the  action  is  now  very 
violent,  hydrogen  gas  is  evolved  as  before,  a  high  tem- 
perature is  developed,  and  the  metal  melts.  If  we  bring 


EXPERIMENT   WITH   SODIUM   AND   WATER.  277 

a  lighted  match  near  the  swimming  globule,  the  escap- 
ing hydrogen  makes  its  presence  evident  by  taking  fire 
and  burning,  although  with  a  peculiar  yellow  flame, 
which  owes  its  color  to  the  presence  of  a  trace  of 
sodium  vapor.  Any  volatile  compound  of  sodiurr 
introduced  into  a  non-luminous  gas-flame  produces  the 
same  effect.  But  where  is  the  other  product  we  are 
seeking  ?  Evidently  we  must  look  for  it  in  the  water, 
on  which  the  sodium  has  been  acting.  Have  the  quali- 
ties of  the  liquid  changed  ?  This  question  can  be  an- 
swered by  a  simple  test.  Here  we  have  some  strips  of 
paper,  which  are  colored  with  certain  well-known  vege- 
table dyes.  The  yellow  strips  are  colored  with  tur- 
meric, and  the  red  with  litmus.  On  dipping  these 
strips  in  a  jar  of  pure  water,  notice  that  the  color  is  not 
in  the  least  degree  modified  ;  but  mark  that,  when  the 
yellow  strip  is  drawn  through  the  water  on  which  the 
sodium  has  been  acting,  the  color  becomes  at  once  bright 
red ;  while,  on  the  other  hand,  the  strip  colored  red  by 
litmus  becomes  blue.  Evidently  it  is  some  product  of 
the  reaction  dissolved  in  the  water  which  produces  these 
changes,  and  this  conclusion  will  be  confirmed  on  tast- 
ing the  water,  which  has  acquired  a  sharp,  biting  taste, 
and  attacks  the  skin,  producing,  when  rubbed  between 
the  fingers,  a  peculiar  unctuous  feeling,  effects  which 
every  one  will  recognize  as  those  of  a  caustic  alkali.  If, 
now,  we  evaporate  the  water,  we  shall  obtain  a  small 
quantity  of  an  amorphous,  white  solid,  similar  to  that 
which  is  contained  in  this  bottle,  and  which  is  only  a 
purer  form  of  the  caustic  soda  of  commerce  used  in 
such  great  quantities  for  making  soap. 

As  we  are  able  to  discover  no  other  results  of  this 
process  except  the  two  substances  you  have  seen,  you 
may  conclude  that  the  only  products  of  the  reaction  of 
sodium  on  water  are  hydrogen  gas  and  caustic  soda. 


278  QUANTIVALENCE  AND   METATHESIS. 

Next,  as  to  the  nature  of  the  process,  and  how  we  can 
express  it  by  our  symbols.  We  know  all  about  the 
molecular  constitution  of  the  factors  of  the  reaction. 
The  symbol  of  a  molecule  of  sodium  is  N"a-Na,  and 
that  of  water  H-O-H.  These  molecules  have  the  sim- 
plest types  of  structure.  We  also  know  that  the  mole- 
cule of  hydrogen  gas  has  the  symbol  H-H,  but  how 
about  the  molecule  of  caustic  soda  (sodic  hydrate,  as  we 
call  it)  ?  Chemical  analysis  shows  that  this  substance 
consists  simply  of  sodium,  oxygen,  and  hydrogen,  in 
proportions,  by  weight,  corresponding  exactly  with 
those  proportions  which  have  been  assumed  to  be  the 
relative  weights  of  the  atoms  of  these  three  elements. 
Analysis,  therefore,  proves  that  the  molecule  of  caustic 
soda  contains  an  equal  number  of  atoms  of  all  three  of 
its  elementary  constituents,  but  it  does  not  enable  us  to 
decide  whether  its  symbol  is  NaOH  or  Na2O2H2,  or  any 
other  simple  multiple  of  these  letters.  Here,  however, 
the  principles  of  quantivalence  come  to  our  aid.  We 
know  that  both  H  and  Na  are  univalent  atoms,  and 
that  the  molecule  of  oxygen  can  only  hold  two  such 
atoms.  Hence  the  symbol  must  be  Na-O-H,  and  can 
be  nothing  else.  Were  caustic  soda  a  volatile  solid,  so 
that  we  could  determine  the  specific  gravity  of  its  va- 
por, we  could  reach  a  knowledge  of  its  molecular  con- 
stitution in  the  manner  previously  described,  which  is 
much  more  direct  and  satisfactory  ;  but,  as  it  cannot  be 
volatilized  within  any  manageable  limits  of  tempera- 
ture, we  are  obliged  to  resort  to  methods  whose  re- 
sults are  undoubtedly  less  conclusive,  and  depend,  to  a 
greater  or  less  degree,  on  theoretical  considerations. 

Writing  out,  now,  the  symbols  of  the  factors  and 
products  of  our  reaction, 

Na-Na  H-O-H  Na-O-H  H-H, 


THE  PROCESS  EXPLAINED.  279 

we  notice  that,  as  there  are  two  atoms  of  Na  in  the 
molecule  of  the  metal,  we  must  have  formed  two  mole- 
cules of  Na-O-H,  and,  as  there  will  then  be  four  atoms 
of  hydrogen  among  the  products,  there  must  be  two 
molecules  of  water  used  in  the  factors,  and  our  reac- 
tion, thus  amended,  becomes 

Na-Na  +  2H-0-H  =  2Na-O-H  +  H-H. 

If,  next,  we  represent  the  reaction  by  graphic  symbols, 
the  nature  of  the  change  will  be  made  still  more  evi- 
dent : 

H-O-H        £Ta        Na-O-H        H 
+     i     =  +i 

H-O-H        Na        Na-O-H        H. 

It  will  be  now  seen  that  the  two  atoms  of  sodium  have 
changed  place  each  with  an  atom  of  hydrogen  in  the 
molecule  of  water,  and  that  the  displaced  atoms  of 
hydrogen  have  taken  the  place  of  the  atoms  of  sodium. 
In  a  word,  the  new  molecules  have  precisely  the  same 
structure  as  the  old,  and  only  differ  from  them  in  the 
substitution  of  Na  for  H,  or  the  reverse.  This  reac- 
tion is,  therefore,  a  simple  example  of  metathesis. 

Caustic  soda  (or  sodic  hydrate),  which  was  one  of 
the  products  of  the  reaction  we  have  been  studying, 
belongs  to  a  class  of  substances  which  have  long  been 
distinguished  for  their  very  marked  and  useful  quali- 
ties, and  are  called  alkalies.  The  most  striking  and 
familiar  of  these  qualities  have  already  been  noticed, 
and,  among  others,  the  effects  which  the  alkalies  produce 
on  the  colored  papers  dyed  with  turmeric  or  litmus. 
Now,  there  is  another  class  of  compounds  whose  quali- 
ties, while  equally  marked,  bear  a  most  striking  antithe- 
sis to  those  of  the  alkalies.  These  compounds  are  called 
acids,  and  the  word  recalls  a  peculiar  taste  and  a  corrosive 
action,  with  which  every  one  is  more  or  less  familiar. 
Here  we  have  one  of  these  substances,  the  muriatic 


280  ALKALIES  AND  ACIDS. 

acid  of  commerce,  which,  as  I  have  already  told  you,  is 
a  solution  of  hydrochloric-acid  gas  (HC1)  in  water. 
Notice  that,  when  I  dip  in  this  acid  solution  the  dyed 
papers  which  have  been  altered  by  the  alkali,  their 
former  color  is  at  once  restored.  The  acid  thus  undoes 
the  effect  of  the  alkali,  and,  what  is  more,  if  I  add  the 
acid  slowly  to  the  alkaline  solution,  and,  after  each 
addition,  test  the  solution  with  my  papers,  I  shall  find 
that  the  alkaline  reaction,  as  we  call  it,  becomes  feebler 
and  feebler  until  at  last  it  wholly  disappears.  So,  on  the 
other  hand,  if  we  add  the  alkaline  to  the  acid  solution, 
the  test-papers  will  show  that  the  acid  qualities  disappear 
in  a  similar  manner,  and  we  can  easily  bring  the  solu- 
tion to  such  a  condition  that  it  has  no  more  effect  on 
the  vegetable  dyes  than  so  much  pure  water.  This 
chemical  process  is  usually  described  by  saying  that  the 
acid  and  alkali  neutralize  each  other,  and  notice  that  in 
the  case  before  us  the  test-papers  show  that  the  neutral 
point  has  been  reached.  On  tasting  the  solution,  we 
cannot  discover  the  least  traces  of  either  an  acid  or  an 
alkaline  taste,  but  in  their  place  we  recognize  the  flavor 
of  common  salt,  and  if  we  evaporate  the  solution  we 
shall  obtain  a  small  quantity  of  this  most  familiar  con- 
diment. 

With  all  the  substances  concerned  in  the  reaction 
we  have  just  studied,  we  are  perfectly  familiar.  Let  us 
see,  then,  if  we  cannot  express  the  reaction  by  means 
of  our  chemical  symbols: 

Ka-O-H         +         HC1         =       H-O-H       +         Na-Cl. 

Sodic  Hydrate.  Hydrochloric  Acid.  Water.  Sodic  Chloride. 

The  reaction  evidently  consists  in  the  simple  substitu- 
tion of  Na  for  H  in  the  molecule  of  HC1,  and  the 
reproduction  of  a  molecule  of  water,  which,  mixing 
with  the  great  mass  of  water  present,  would  naturally 
be  lost  sight  of  in  the  experiment. 


ACTION  OF  POTASSIUM  ON  WATER.  281 

It  appears,  then,  that,  in  the  present  case  at  least, 
the  neutralizing  of  an  acid  by  an  alkali  is  a  simple 
metathetical  reaction,  in  which  the  metallic  atom  of 
the  alkaline-molecule  changes  place  with  the  hydrogen- 
atom  of  the  acid-molecule.  Now,  the  chief  interest  of 
this  experiment  arises  from  the  fact  that  it  is  a  single 
example  of  a  general  truth,  and  the  principle  is  one 
of  such  importance  that  it  requires  further  illustration. 

On  the  second  pan  of  water  I  therefore  throw  a 
lump  of  another  metallic  element,  closely  allied  to  so- 
dium, called  potassium.  The  action  is  even  more  vio- 
lent than  before,  and  mark  that  the  escaping  hydrogen 
inflames  while  the  metallic  globule  is  swimming  rapidly 
about  on  the  surface  of  the  water.  Notice,  also,  the 
beautiful  color  which  the  potassium-vapor  imparts  to 
the  flame,  so  different  from  that  obtained  with  sodium. 
These  colors  are,  in  fact,  very  characteristic,  and,  when 
examined  with  the  spectroscope,  are  condensed  in  cer- 
tain luminous  bands,  whose  positions  on  the  scale  of 
the  instrument  afford  a  never-failing  indication  of  the 
presence  of  the  metal  in  the  flame.  You  see,  more- 
over, that,  as  before,  the  water  has  acquired  an  alkaline 
reaction,  and,  if  we  evaporate  the  solution,  we  shall  ob- 
tain a  small  quantity  of  a  white  solid  called  potash  (or 
potassic  hydrate),  so  similar  to  caustic  soda  that  the  two 
can  scarcely  be  distinguished  except  by  chemical  tests. 
The  process  is  so  analogous,  in  every  respect,  to  the 
last,  that  it  is  certainly  unnecessary  to  repeat  the  evi- 
dence on  which  our  knowledge  of  the  reaction  is 
based,  but  we  will  express  it  at  once  by  our  chemical 
symbols : 

K-K        +        2H-O-H        =        H-H        +        2K-O-H. 

Potassium.  Water.  Hydrogen  Gas.  Potassic  Hydrate. 

The  sole  difference  is  that  we  have  here  atoms  of  potas- 

20 


282  ALKALIES  AND  ACIDS. 

slum,  K,  instead  of  atoms  of  sodium,  Na,  which,  how- 
ever, like  the  last,  take  the  place  each  of  a  hydrogen- 
atom  in  one  of  the  molecules  of  water. 

In  the  previous  example  we  neutralized  the  alkali 
soda  with  hydrochloric  acid.  We  have  here  another 
compound  of  the  same  class,  called  nitric  acid,  and  let 
us  see  whether,  in  like  manner,  this  acid  will  neu- 
tralize the  alkali  potash.  Notice  that,  as  we  add  the 
acid,  the  alkaline  reaction  becomes  feebler  and  feebler, 
until  at  last  it  has  entirely  disappeared.  The  liquid 
has  now  no  effect  on  either  of  these  sensitive  papers. 
On  tasting  it,  we  discover  no  pungency,  and  likewise 
no  acidity,  but  we  recognize  a  peculiar  saline  taste, 
which  is  not  unfamiliar.  Here  is  a  bit  of  paper  which 
has  been  dipped  in  a  similar  solution  and  dried.  See 
how  it  sparkles  when  lighted,  and  every  boy  will  tell  us 
that  we  are  dealing  with  the  well-known  salt  we  call 
nitre.  And  so  it  is ;  and,  on  evaporating  the  solution, 
we  should  obtain  the  familiar  crystals  of  this  substance. 

Before  we  can  explain  this  new  reaction,  we  must 
know  what  is  the  symbol  of  a  molecule  of  nitric 
acid,  and  also  that  of  a  molecule  of  nitre.  Since  nei- 
ther of  these  substance's  can  be  volatilized  without  de- 
composition, we  cannot  weigh  their  vapor,  and  cannot 
therefore  apply  the  method  of  finding  the  symbol  we 
explained  in  a  previous  lecture.  As  in  the  case  of  the 
sodic  hydrate,  however,  we  are  not  wholly  helpless,  for 
analysis  will  tell  us  a  great  deal,  and,  once  for  all,  let  r_s 
consider  just  how  much  information  an  accurate  analy- 
sis will  give  us  in  regard  to  the  symbol,  and  how  far 
it  leaves  us  in  the  dark. 

Here,  then,  we  have  the  analysis  of  nitric  acid,  and 
in  regard  to  the  accuracy  of  these  numbers  there  can- 
not be  a  doubt : 


SYMBOL  OF  NITRIC  ACID.  283 

Hydrogen 1.59          1        H 

Nitrogen.. 22.22        14        N 

Oxygen 76.19        48        O3 

100 

Nitric  acid  consists  of  the  three  elementary  substances 
— hydrogen,  nitrogen,  and  oxygen — in  the  exact  pro- 
portions here  indicated,  just  so  many  per  cent,  of 
each.  Now,  these  per  cents,  are  to  each  other  pre- 
cisely as  the  numbers  1  :  14  :  48  ;  or,  as  the  weight  of 
one  atom  of  hydrogen  is  to  the  weight  of  one  atom  of 
nitrogen  is  to  the  weight  of  three  atoms  of  oxygen  ;  or, 
in  symbols,  as  H  :  N  :  O3.  But,  as  every  one  knows, 
we  may  multiply  all  the  terms  of  a  proportion  by  any 
number  we  please  without  in  the  least  altering  the 
value  of  the  ratios — thus,  1  :  14  :  48  =  H  :  1ST  :  O3  = 
H2  :  N2  :  O6  =  H3  :  N3  :  O9 ;  or,  in  general,  as  Hn  : 
Nn  :  O3n.  Hence,  then,  if  nitric  acid  consists  of  hy- 
drogen, nitrogen,  and  oxygen,  in  the  proportions  which 
our  analysis  indicates,  its  molecule  must  be  represented 
either  by  HNO3,  or  by  some  simple  multiple  of  these 
symbols.  Knowing,  then,  as  we  do,  the  relative 
weights  of  the  atoms,  simple  analysis  will  tell  us  in 
every  case  the  relative  number  of  atoms  present  in 
the  molecule,  but  it  cannot  fix  the  absolute  number. 

You  see,  therefore,  that  analysis  alone  gives  us  al- 
ways a  close  approximation  to  the  symbol,  and  limits 
the  question  within  very  restricted  bounds.  The  sim- 
plest formula  in  any  case  is  that  which  represents  the 
molecule  as  consisting  of  the  smallest  number  of  whole 
atoms  which  will  satisfy  the  conditions,  and  the  only 
question  can  be  as  between  this  symbol  and  its  multi- 
ples. In  the  case  of  all  volatile  compounds,  a  very 
rough  determination  of  their  vapor  density  is  sufficient 
to  decide  the  question.  Thus,  in  the  case  of  nitric  acid, 


284  ALKALIES  AND  ACIDS. 

if  the  symbol  is  HNO3,  the  molecular  weight  is  63, 
and  the  vapor  density  would  be  31.5.  Were  the  sym- 
bol H2N2O6,  the  density  would  be  63  ;  were  it  H3N3O9, 
the  density  would  be  94.5  ;  and,  although  there  are 
causes  which  make  many  of  our  determinations  of  va- 
por densities  untrustworthy  within  several  per  cent., 
they  are  abundantly  accurate  enough  to  show  which  of 
such  widely-differing  values  must  be  the  true  one. 

Hence,  although  theoretically  the  molecular  weight, 
as  determined  by  the  vapor  density,  is  our  starting- 
point  in  the  investigation  of  the  symbol  of  a  compound, 
practically  it  is  only  used  to  control  the  results  of  anal- 
ysis. So  also,  when,  in  the  case  of  non  volatile  com- 
pounds, we  must  resort  to  other  modes  of  fixing  the 
molecular  weight,  an  accurate  analysis  having  once  been 
made,  the  question  lies  only  between  a  few  widely-differ- 
ing numbers,  and  considerations  are  sufficient  to  decide 
between  these  which  w^ould  not  be  regarded  as  satisfac- 
tory were  greater  accuracy  required. 

Of  course,  as  must  be  expected,  there  are  substances 
in  regard  to  which  no  definite  conclusions  can  be 
reached,  and  where  conflicting  evidence  renders  differ- 
ences of  opinion  possible.  This  is  true  of  many  min- 
eral species,  and  the  symbols  of  such  compounds  are  in 
doubt  to  the  extent  I  have  mentioned.  In  such  cases, 
we  usually  adopt  provisionally  the  simplest  symbol 
which  the  analytical  results  permit,  and  wait  for  the 
advance  of  science  to  correct  any  error  which  may  be 
made,  and  which,  for  the  time  at  least,  is  unimportant. 

When  it  is  remembered  that  the  molecular  weight 
of  a  substance  can  always  be  calculated  from  its  symbol, 
it  is  obvious  from  the  above  example  that  an  accurate 
analysis  always  gives  us  the  means  of  determining  the 
smallest  possible  molecular  weight,  and  the  true  value 


DIFFERENCES   OF   METHOD.  285 

must  be  either  the  value  thus  determined,  or  some  sim- 
ple multiple  of  it.  Thus,  in  the  case  of  nitric  acid,  the 
molecular  weight  must  be  either  03  or  a  simple  multi- 
ple of  63.  In  a  previous  lecture  we  saw  that,  with  the 
same  limitations,  an  exact  value  of  the  molecular  weight 
might  be  derived  from  the  definite  proportions  which 
a  substance  preserves  in  any  of  the  chemical  processes 
into  which  it  enters,  and  we  made  these  definite  pro- 
portions the  basis  from  which,  in  theory,  we  derived  the 
accurate  values  of  the  molecular  magnitudes  on  which 
We  built  the  system  of  chemical  symbols  and  formulas. 
But  practically  we  do  not  directly  determine  the  com- 
bining proportions^  because  the  same  knowledge  may  be 
deduced  from  the  results  of  the  analysis,  which  it  is 
always  our  first  object  to  make  in  studying  the  chemical 
relations  of  every  new  substance ;  and  we  now  see  that 
from  the  results  of  an  accurate  analysis  we  can  always 
calculate  the  simplest  possible  symbols  and  the  smallest 
possible  molecular  weight,  and,  with  these  data,  we  can 
always  predict  what  must  be  the  definite  proportions  in 
any  process  where  the  reaction  is  known.  Still,  although 
practically  We  calculate  these  definite  proportions  in- 
stead of  determining  them  experimentally^  yet  it  is  per- 
fectly possible  to  reverse  the  process  as  we  did  reverse 
the  reasoning ;  and  our  method  of  presenting  the  facts 
had  the  great  advantage  of  giving  us  clear  conceptions 
of  the  first  principles  of  our  chemical  philosophy  before 
we  had  any  knowledge  of  chemical  symbols  or  of  the 
results  of  analysis  which  they  express. 

This  is,  undoubtedly,  a  difficult  subject — one  of  the 
most  difficult  in  chemistry ;  but  the  difficulty  can  be 
mastered  with  a  little  thought,  and  it  requires  no  de- 
tailed knowledge  of  the  science  to  follow  the  reasoning 
thus  far.  It  is  different,  however,  with  the  purely  chem- 


286  ALKALIES  AND   ACIDS. 

ical  evidence  on  which  we  are  frequently  obliged  to 
rely  for  deciding  between  the  few  formulas  which,  in  a 
given  case,  analysis  shows  may  be  possible.  This  evi- 
dence will  have  no  force,  except  with  those  who  have 
already  a  competent  knowledge  of  the  facts.  Thus  much, 
however,  can  be  understood.  The  facts  of  chemistry, 
like  those  of  any  other  science,  are  parts  of  a  general 
plan  more  or  less  fully  apprehended  by  the  student,  and 
the  evidence  of  which  I  am  speaking  may  be  summed 
up  in  the  statement  that  the  given  symbol  is  accepted 
because  it  is  consistent  with  this  plaii.  Of  course,  such 
reasoning  is  not  absolutely  conclusive,  and  there  is 
room  for  doubt,  but  so  there  is  in  every  department  of 
science.  A  part  of  the  way  we  walk  in  the  clear  light 
of  knowledge  ;  the  rest  of  the  way  we  grope  ;  but  it  is 
only  thus  that  we  can  penetrate  the  darkness  of  the 
unknown,  and  we  rely  on  that  intelligence  in  man 
which  finds  its  response  in  the  intelligence  of  Nature, 
to  direct  our  steps. 

Having  now  explained  as  fully  as  our  time  will  per- 
mit the  general  nature  of  the  evidence  on  which  we 
depend  for  establishing  the  empirical  symbol  of  a  com- 
pound, I  shall  not  recur  to  it  again,  but  shall  regard  it 
as  sufficient  to  say  that  chemists  are  agreed  that  the 
symbol  is  thus  or  so.  In  the  case  of  nitric  acid,  there 
is  no  question  that  the  symbol  of  the  molecule  is 
HNO3,  and,  in  like  manner,  KNO3  is  the  received  sym- 
bol of  nitre.  How,  now,  shall  we  write  the  reaction  we 
last  studied  ?  Simply  thus  : 

K-O-H       +       H-N03       =      H-O-H       +       K-STOs. 

Potassic  Hydrate.  Nitric  Acid.  Water.  Potassic  Nitrate. 

The  reaction,  then,  consists  merely  in  an  interchange 
between  the  hydrogen-atom  of  the  acid  and  the  metal- 
lic atom  of  the  alkali.  It  is,  then,  precisely  similar  to 


CHARACTERISTICS   OF   AN   ACID.  287 

the  reaction  between  sodic  hydrate  and  hydrochloric 
acid;  and,  if,  as  I  said  before,  these  are  only  examples 
of  what  is  true  in  the  case  of  all  alkalies  and  all  acids, 
we  are  certainly  justified  in  deducing  from  our  experi- 
ments the  following  principles :  First,  an  alkali  is  a 
substance  whose  molecules  have  a  definite  structure, 
and  differ  from  the  molecules  of  water  only  in  hav- 
ing a  metallic  atom  in  place  of  one  of  the  hydrogen- 
atoms  of  the  water-molecule ;  secondly,  an  acid  is  a 
substance  whose  molecules  contain  at  least  one  atom 
of  hydrogen,  which  is  readily  replaced  by  the  metallic 
atom  of  the  alkali  when  the  two  substances  are  brought 
together. 

As  the  illustrations  already  given  indicate,  the  char- 
acteristic qualities  of  an  acid  depend  upon  the  circum- 
stance that  certain  hydrogen -atoms  in  the  molecules 
of  these  substances  are  readily  replaced  by  metallic 
atoms.  In  my  next  lecture,  I  shall  show  that  this  sus- 
ceptibility to  replacement  depends  upon  a  definite 
molecular  structure,  but  I  must  not  leave  this  subject 
without  insisting  on  the  fact  that  this  characteristic  of 
acids  is  manifested  in  other  ways  besides  the  special 
mode  we  have  been  studtying.  A  few  experiments  will 
illustrate  this  point : 

In  this  flask  there  are  some  wrought-iron  nails. 
"We  pour  over  them  some  muriatic  acid,  and  warm  the 
vessel.  At  once  there  is  a  brisk  evolution  of  gas,  which 
we  are  here  collecting,  in  the  usual  way,  over  water ; 
and  notice  that,  when  lighted,  the  gas  burns  with  the 
familiar  flame  of  hydrogen.  Muriatic  acid  is  an  old 
friend.  We  know  all  about  its  constitution,  and  it 
is  evident  that  the  iron-atoms  have  replaced  the  hy- 
drogen-atoms of  the  acid.  If  we  evaporate  the  solu- 
tion left  in  the  flask,  we  shall  obtain  a  green  salt  con- 


288 


ALKALIES  AND  ACIDS. 


sisting  of  chlorine  and  iron.      The  reaction  is  thus 
represented  : 

H-C1 

+        Fe         =        Fe 
H-C1 

Hydrochloric  Acid.  Iron. 


oi 


H 


Ferrous  Chloride. 


H. 

Hydrogen  Gas. 


As  the  iron-atom  is  bivalent,  it  takes  the  place  of  two 
atoms  of  hydrogen,  which,  when  thus  displaced,  form  a 
molecule  of  hydrogen  gas. 


FIG.  31.— Preparation  of  Hydrogen  Gas. 

In  the  second  flask  are  some  zinc-clippings,  and  we 
will  pour  over  them  some  dilute  sulphuric  acid,  one  of 
the  best  known  of  the  class  of  compounds  we  are  study- 
ing. Again,  notice  a  brisk  evolution  of  gas  (Fig.  31), 
which  also,  as  you  see,  burns  like  hydrogen.  Indeed, 
this  is  the  process  by  which  hydrogen  gas  is  usually 
made : 


H2=SO4 

Sulphuric  Acid. 


Zn 

Zinc. 


=       Zn=SO4 

Zinc  Sulphate. 


+       H-H. 

Hydrogen  Gas. 


In  the  reaction,  which  is  here  written,  you  notice  that, 
as  before,  the  metallic  atom  takes  the  place  of  two 
atoms  of  hydrogen;  but  sulphuric  acid  differs  from 


SUSCEPTIBILITY  TO  SUBSTITUTION.  289 

both  hydrochloric  acid  and  nitric  acid  in  that  each  of 
its  molecules  has  two  atoms  of  hydrogen,  which  can  be 
thus  replaced. 

Examples  like  these  might  be  multiplied  indefi- 
nitely. We  will  conclude,  however,  with  one  more 
experiment,  which  illustrates  the  same  susceptibility  to 
substitution,  but  under  slightly  different  conditions. 
This  white  powder  is  called  zinc  oxide,  and  is  a  com- 
pound of  zinc  with  oxygen.  Notice  that  it  dissolves 
readily  in  a  portion  of  the  same  dilute  sulphuric  acid 
used  in  the  last  experiment.  Moreover,  on  evaporating 
the  solution,  we  should  obtain  zinc  sulphate  (ZnSO4), 
the  same  product  as  before.  Why,  then,  is  there  no 
hydrogen  gas  evolved  ?  Let  our  symbols  tell  us : 

ZnO       +       H2SO4       =      H2O       +       ZnS04. 

Zinc  Oxide.          Sulphuric  Acid.  Water.  Zinc  Sulphate. 

You  see  that  the  metathesis  yields  water  instead  of 
hydrogen  gas,  and  the  question  is  answered. 


LECTURE  XIII. 

ELECTRO-CHEMICAL   THEORY. 

IN  our  last  lecture  we  saw  that,  whether  an  acid  is 
brought  in  contact  with  an  alkali,  a  metal,  or  a  metallic 
oxide,  on«  or  more  of  the  hydrogen-atoms  in  its  mole- 
cules become  replaced  by  metallic  atoms  from  the  mole- 
cules of  the  associated  body,  and  this  susceptibility  to 
replacement  was,  as  I  stated,  the  distinguishing  feat- 
ure of  that  class  of  compounds  we  call  acids.  But  I 
should  leave  you  with  a  very  imperfect  notion  of  these 
important  relations,  if  I  did  not  proceed  further  to 
illustrate  that  the  class  of  compounds  we  call  alkalies, 
and  which  we  have  been  accustomed  to  regard  as  the 
very  opposite  of  acids,  have  exactly  the  same  charac- 
teristics. 

In  this  small  glass  flask  there  are  some  clippings  of 
the  metal  aluminum,  the  metallic  base  of  clay  which 
has,  within  a  few  years,  found  many  useful  applica- 
tions in  the  arts.  On  this  metal  I  pour  a  solution  of 
caustic  potash.  Notice  that,  on  heating  the  flask,  I 
obtain  a  brisk  evolution  of  gas.  On  lighting  the  gas,  it 
burns  with  a  flame  which  leaves  us  no  doubt  that  the  gas 
is  hydrogen.  What,  now,  is  the  reaction  ?  Somewhat 
more  complex  than  those  you  have  previously  studied, 


A  POINT   OF   RESEMBLANCE.  291 

because  the  atom  of  aluminum  has  a  quanti valence  of 
six.  Moreover,  in  order  to  satisfy  certain  very  striking 
analogies,  we  write  the  symbol  of  this  atom  A12,  that 
is,  we  take  27.4  m.c.  of  aluminum  for  the  assumed 
atom,  and  represent  that  by  Al,  although  54.8  m.c., 
which  we  write  A12,  is  the  smallest  quantity  of  the  ele- 
ment of  which  we  have  any  knowledge,  or  which 
changes  place  with  other  atoms  in  the  numerous  meta- 
thetical  reactions  with  which  we  are  acquainted.  Here 

K-O-H 
K-O-H 

i  I  o  -  H  +  Ala  =  K«vi°«vi  A1*  +  3H-H- 

K-O-H 
K-O-H 

the  Ala  takes  the  place  of  six  hydrogen-atoms,  thus 
binding  together  what  were  before  six  distinct  mole- 
cules of  K-O-H  into  a  single  molecule  of  the  resulting 
product.  Evidently,  then,  the  hydrogen- atom  in  the 
molecule  of  the  alkali  has  the  same  facility  of  re- 
placement as  that  in  the  molecule  of  the  acid.  Nor 
is  this  an  isolated  example,  although,  perhaps,  the  most 
striking  we  could  adduce,  and  it  illustrates  a  truth 
which  was  recognized  long  before  the  general  adoption 
of  the  new  philosophy  of  chemistry.  Acids  and  alka- 
lies belong  to  the  same  class  of  compounds,  and  caustic 
potash  and  nitric  acid  are  simply  the  opposite  extremes 
of  a  series  of  bodies  in  which  all  the  intermediate 
gradations  are  fully  represented.  In  our  modern 
chemistry  we  call  this  class  of  chemical  substances  hy- 
drates, and  we  distinguish  the  two  extremes  of  the 
class  as  alkaline  (or  basic)  and  acid  hydrates,  respec- 
tively. The  terms  alkaline  and  basic  are  here  used 
synonymously,  although  the  first  is  generally  restricted 
to  the  old  caustic  alkalies,  including  ammonia  and  the 


292  ELECTRO-CHEMICAL  THEORY 

few  compounds  closely  allied  to  them,  which  have  been 
recently  discovered* 

Seeing,  now5  that  the  hydrogen-  atom  in  the  mole- 
cule of  potassic  hydrate  has  the  same  susceptibility  of 
replacement  whose  cause  we  are  seeking  to  discover, 
and  knowing,  as  we  do,  the  structure  of  this  alkaline 
molecule,  may  it  not  be  a  similar  structure  which  de- 
termines the  like  susceptibility  in  the  molecules  of  all 
acids  ;  for  example^  in  those  of  nitric  acid  ?  What,  now, 
is  the  position  of  the  hydrogen  -atom  in  the  molecule 
which  we  have  so  often  written,  K-O-H  ?  Why,  simply 
this*  It  is  one  end  of  a  chain  of  three  atoms  which 
has  an  atom  of  the  metal  potassium  at  the  other  end, 
and  an  atom  of  oxygen  connecting  the  two*  Now,  we 
can  Write  the  symbol  of  nitric  acid  thus  : 


and  you  will  observe  that  we  thus  satisfy  all  the  condi- 
tions of  quantivalence,  and  have  a  structure  similar  to 
that  of  potassic  hydrate.  As  before,  we  have  an  atom 
of  oxygen  uniting  the  hydrogen  atom  writh  the  other 
end  of  the  chain  ;  but  then  this  end  of  our  molecular" 
structure  is  formed,  not  by  a  single  atom,  but  by  a 
group  of  atoms  (NO2)  which,  nevertheless,  can  be  re^ 
placed  by  metathesis  just  like  a  simple  atom. 

Allow  me  here,  however,  to  make  a  short  digres- 
sion from  the  main  line  of  my  argument,  in  order  to 
define  an  important  term  which  we  shall  have  frequent 
occasion  to  use  during  this  lecture.  By  comparing  the 
symbol  K-O-H  with  H-O-(NO2),  it  will  be  evident  that 
the  only  essential  difference  between  them  is  that  the 
group  ]STO2  in  the  last  takes  the  place  of  the  atom  K  in 
the  first.  It  must  be,  then,  the  influence  of  this  part 
of  the  molecule  which  determines  the  difference  be- 


COMPOUND   RADICALS.  293 

tween  a  strong  alkali  and  a  strong  acid.  Now,  such  an 
atom  or  such  a  group  of  atoms,  which  appears  to  deter- 
mine the  character  of  the  molecule,  is  constantly  called 
in  chemistry  a  radical.  Thus  K  is  the  radical  of  the 
molecule  K-O-H,  and  NO2  the  radical  of  the  molecule 
H-O-NO2 ;  but,  while  the  potassium-atom  is  called  a  sim- 
ple radical,  the  group  NO2  forms  what  is  known,  as  a 
compound  radical.  The  influence  of  simple  radicals  in 
determining  the  qualities  of  their  compounds  has  long 
been  recognized.  Indeed,  the  old  chemistry  laid  alto- 
gether too  much  stress  on  this  influence,  regarding  the 
qualities  of  a  substance  as  derived  in  some  unknown 
but  remote  manner  from  the  qualities  of  its  elements, 
and  wholly  ignoring  the  effect  of  molecular  structure 
on  these  qualities,  which  we  now  know  to  be  at  least 
equally  great.  It  was  a  very  great  step  forward  when 
the  German  chemist  Liebig  first  recognized  the  truth 
that  a  group  of  atoms  might  give  a  distinctive  charac- 
ter to  a  class  of  compounds  just  as  effectively  as  an  ele- 
mentary atom.  These  groups  he  first  named  com- 
pound radicals,  and  assigned  some  of  the  names  by 
which  the  more  important  of  them  are  stil]  known, 
and  we  now  speak  just  as  familiarly  of  the  compounds 
of  cyanogen  (CN),  of  ammonium  (N"H4),  of  methyl 
(CH3),  of  ethyl  (C2H5),  etc.,  as  we  do  of  the  compounds 
of  chlorine,  potassium,  zinc,  or  iron.  Moreover,  each 
of  the  compound  radicals,  like  a  simple  radical,  has 
a  definite  quanti valence,  but,  while  the  quant i valence 
of  the  simple  radical  depends  on  wholly  unknown 
conditions,  that  of  the  compound  radical  depends  on 
the  quantivalence  of  the  elementary  atoms  of  which  it 
consists.  Thus  the  radical  NO2  is  univalent  because 
one  only  of  the  five  bonds  of  the  nitrogen-atom  re- 
mains unclosed,  as  the  symbol  indicates.  Examine, 


294  ELECTRO-CHEMICAL   THEORY. 

also,  the  graphic  symbols  of  the  other  compound  radi- 
cals mentioned  above : 

H    H  H  H  H 

V-  -(0=N)          H-C-         H-c'-C- 

/  \  i  ii 

H    H  H  H  H 

Ammonium.  Cyanogen.  Methyl.  Ethyi 

In  each  case,  the  number  of  bonds  which  are  not  closed 
determines  the  quanti valence. 

Returning,  now,  to  our  comparison  between  K-O-H 
and  H-O-NO2,  we  should  describe  the  relations  of  the 
molecules  in  a  few  words  by  saying  that  the  acid  and 
the  alkali  had  molecules  of  the  same  general  structure, 
but  differed  in  that  the  radical  of  the  alkali  was  the  ele- 
mentary atom  potassium,  while  the  radical  of  the  acid 
was  the  atomic  group  NO2. 

As  the  result,  then,  of  our  discussion,  we  are  led  to 
the  theory  that  acids  and  alkalies  are  compounds  hav- 
ing the  same  general  molecular  structure,  and  that  the 
susceptibility  to  replacement  of  the  hydrogen-atom  or 
atoms,  which  all  these  compounds  contain,  depends 
upon  the  molecular  structure,  while  the  differences  be- 
tween acids  and  alkalies,  and,  we  might  add,  the  differ- 
ences between  individual  acids  or  individual  alkalies,  de- 
pends on  the  nature  of  the  radical.  Having  been  led 
thus  far,  the  question  next  arises,  Can  we  trace  any  con- 
nection between  the  acid  and  alkaline  characters  of  the 
compounds,  on  the  one  side,  and  the  nature  of  the  rad- 
icals, which  appear  to  determine  these  features,  on  the 
other  side  ? 

The  simple  radicals,  as  they  appear  in  the  elemen- 
tary substances,  may  be  divided  into  two  great  classes, 
the  metals  and  the  non-metals,  the  last  class,  by  a  sin- 
gular perversion  of  language,  being  frequently  called 


POSITIVE  AND   NEGATIVE   RADICALS.  295 

metalloids.  Now,  the  most  elementary  knowledge  of 
chemistry  shows  that,  while  radicals  of  opposite  na- 
tures combine  most  eagerly  together,  two  metals,  or  two 
closely-allied  metalloids,  show  but  little  affinity  for  each 
other.  These  facts  suggest  at  once  an  analogy  between 
chemical  affinity  and  the  familiar  manifestations  of 
polar  forces  in  electricity  and  magnetism  ;  where  it  is 
also  true  that  the  like  attracts  the  unlike.  Moreover, 
it  is  found  that,  when,  in  the  various  processes  of  elec- 
trolysis, chemical  compounds  are  decomposed  by  the 
electrical  current,  the  different  elementary  substances 
appear  at  different  poles  of  the  electrical  combina- 
tion. Thus,  hydrogen,  potassium,  and,  in  general,  the 
metals,  are  evolved  at  what  is  called  the  negative  pole, 
while  oxygen,  chlorine,  bromine,  and  the  allied  metal- 
loids, appear  at  the  positive  pole.  It  was,  then,  not  un- 
natural to  refer  these  effects  of  electrolysis  to  the  elec- 
trical condition  of  the  atoms,  and  to  assume  that  the 
atoms  had  an  opposite  polarity  to  that  of  the  poles,  to 
which  they  were  attracted,  and  hence  the  metals  came 
to  be  called  electro-positive  and  the  metalloids  electro- 
negative radicals;  and  these  facts  were  thought  very 
greatly  to  confirm  the  notion  that  chemical  affinity  is  a 
manifestation  of  polar  force  closely  allied  to  electrical 
attraction. 

As  expounded  by  the  great  Swedish  chemist,  Berze- 
lius,  this  electro-chemical  theory  gave  new  life  to  that 
system  of  chemistry  which,  introduced  into  the  science 
by  Lavoisier  and  his  contemporaries,  has  been  only  re- 
cently superseded.  Corresponding  to  the  duality  of  the 
electrical  and  magnetic  poles,  it  was  argued  that  there 
must  be  a  duality  in  all  chemical  compounds,  the  ele- 
ments uniting  by  twos  to  form  binary  compounds,  the 
binaries  again  uniting  by  twos  to  form  ternary  com- 


296  ELECTRO-CHEMICAL   THEORY. 

pounds,  and  so  on ;  and  from  this,  its  most  character- 
istic feature,  the  old  philosophy  is  now  called  the  dual- 
istic  system.  As  the  knowledge  of  chemical  compounds 
has  been  enlarged,  it  has  been  found  that,  whatever  may 
be  the  resemblances  between  electrical  and  chemical  at- 
traction, the  analogy  fails  in  the  very  point  on  which  the 
dualistic  system  relied.  But  the  chemists  of  the  new 
school,  in  their  reaction  from  dualism,  have  too  much 
overlooked  the  electro -chemical  facts,  which  are  as  true 
now  as  they  ever  were.  The  distinction  between  posi- 
tive and  negative  radicals,  based  on  their  electrical  rela- 
tions, is  evidently  a  most  fundamental  distinction,  al- 
though, as  Berzelius  himself  showed,  the  distinction  is  a 
relative  and  not  an  absolute  one.  It  is  possible  to  clas- 
sify the  radicals  in  one  or  more  series  in  which  any  mem- 
ber is  positive  toward  all  that  follow  it,  and  negative 
toward  all  that  precede  it  in  the  same  series,  and  this 
principle  is  as  true  of  the  compound  as  it  is  of  the  sim- 
ple radicals.  Xow,  it  is  in  this  difference  between  posi- 
tive and  negative  radicals  that  we  shall  find  the  origin 
of  the  distinctive  features  of  the  acid  and  the  alkali. 

Compare,  again,  the  symbols  of  potassic  hydrate 
and  nitric  acid  as  we  have  now  learned  to  write  them — 
K-O-H  and  H-O-N"O2— and  seek,  by  the  electro-chemi- 
cal classification,  to  determine  what  are  the  electrical  re- 
lations of  the  radicals  K  and  NO2,  to  which,  as  I  have 
s,iid,  we  must  refer  the  distinctive  features  of  these 
compounds.  It  will  appear  that  K,  the  radical  of  the 
alkali,  is  the  most  highly  electro-positive,  and  NO2,  the 
radical  of  the  acid,  one  of  the  most  highly  electro-nega- 
tive of  all  known  radicals.  Moreover,  if  you  will  ex- 
tend your  study,  and  compare  in  a  similar  manner  the 
electrical  relations  of  the  other  well-marked  alkaline  and 
acid  hydrates,  you  will  find  that  the  radicals  of  the  al- 


POSITIVE  AND  NEGATIVE   RADICALS.  297 

kalies  are  all  electro-positive,  and  the  radicals  of  the 
acids  all  electro-negative,  and,  further,  that  the  distinc- 
tive features  of  the  alkali  or  the  acid  are  the  more 
marked  in  just  the  proportion  that  the  position  of  the 
radical  of  the  compound,  in  the  electrical  classification, 
is  the  more  extreme.  Lastly,  those  hydrates  whose 
properties  are  indifferent,  and  which  sometimes  act  as 
acids  and  sometimes  as  alkalies,  will  be  found  to  contain 
radicals  occupying  an  intermediate  position  in  the  same 
classification. 

In  following,  then,  the  path  which  theoretical  con- 
siderations have  opened,  wre  have  met  with  a  most  re- 
markable class  of  facts.  Alkalies  contain  radicals  which, 
in  the  process  of  electrolysis,  are  attracted  toward  the 
negative  pole  of  the  battery,  while  acids  contain  radi- 
cals which,  under  the  same  conditions,  are  drawn  toward 
the  positive  pole,  and,  in  the  proportion  as  the  energy 
thus  mutually  exerted  between  radical  and.  pole  is  the 
more  marked,  the  acid  or  alkaline  features  of  the  hy- 
drates of  the  radical  are  the  more  pronounced.  Here 
are  the  facts,  which  no  one  will  question  ;  and  what, 
now,  is  the  explanation  of  them  ?  We  can  give  only  a 
theoretical  explanation  based  on  the  analogies  of  polar 
forces,  a  mode  of  manifestation  of  energy  of  which  the 
chemical  force  appears  to  partake,  as  the  very  phenom- 
ena of  electrolysis  indicate. 

If  we  carefully  study  what  we  have  called  the  dis- 
tinctive features  of  acids  and  alkalies,  they  will  be  found 
to  depend  on  this,  that  the  hydrogen-atoms  of  acids  are 
readily  replaced  only  by  positive,  and  the  hydrogen- 
atoms  of  alkalies  only  by  negative  radicals.  In  other 
words,  with  every  hydrate  the  power  of  easily  replacing 
its  hydrogen-atom  is  only  enjoyed  by  those  radicals 

which  are  opposite  in  their  electrical  relations  to  the 
21 


298  ELECTRO-CHEMICAL  THEORY. 

radical  which  the  hydrate  already  contains.  This  will 
be  found  to  be  the  one  characteristic  to  which  all  that 
is  peculiar  to  either  acid  or  alkali  can  be  referred,  and 
if  we  can  explain  this  we  have  explained  all. 

The  explanation  we  would  offer  is  as  follows  :  The 
oxygen-atom  with  its  two  bonds,  -O-,  is  in  a  condition 
similar  to  that  of  a  bar  of  soft  iron,  susceptible  of  mag- 
netism. When  we  unite  the  atom  by  one  of  these 
bonds  with  a  positive  radical,  we  produce  an  effect  sim- 
ilar to  that  obtained  by  placing  in  contact  with  one  end 
of  such  an  iron  bar  a  powerful  magnetic  pole.  Under 
these  conditions,  as  is  well  known,  the  two  ends  of  the 
bar  become  strongly  polar,  the  farther  extremity  ac- 
quiring a  polarity  of  the  same  kind  as  that  of  the  active 
pole ;  and  so,  in  the  case  of  our  oxygen-atom,  a  posi- 
tive radical  united  at  one  bond  seems  to  polarize  the 
atomic  mass,  and  make  a  positive  pole  at  its  other  end. 

Magnet.  Bar. 


+     —  + 

K-O- 


Furthermore,  if  we  bring  a  lump  of  nickel  in  contact 
with  the  free  pole  of  an  iron  bar,  in  the  condition  thus 
described,  magnetic  attraction  will  be  developed  in  the 
mass  of  the  nickel,  a  negative  pole  will  be  formed  at  the 
point  of  contact,  and  the  lump  will  adhere.  So,  also, 
we  may  suppose  that  a  similar  effect  is  produced  on  the 
somewhat  indifferent  hydrogen-atom,  which,  added  to 
K-O-,  makes  up  the  alkaline  molecule — 

+  -+   - 
K-O-H. 

Lastly,  if  we  bring  near  the  now  loaded  pole  of  our 
iron  bar — to  which  we  will  assume  there  is  attached 
as  large  a  lump  of  nickel  as  it  is  capable  of  holding 


MAGNETIC  ANALOGIES.  299 

—  a  lump  of  soft  iron,  the  pole  will  drop  the  nickel 
and  take  the  iron.  In  like  manner,  if  we  bring  near 
our  alkaline-molecule  a  radical,  like  NO2,  which  has, 
by  its  own  nature,  or  is  capable  of  receiving  by  induc- 
tion, a  higher  degree  of  negative  polarity  than  the  hy- 
drogen-atom, then  the  molecule  drops  the  hydrogen- 
atom  and  takes  the  radical. 

Again,  start  with  the  same  oxygen  -atom  with  its 
two  possible  poles,  and  unite  it  by  one  of  its  bonds  to 
a  negative  radical,  it  is  evident  that  an  opposite  effect 
will  be  produced  to  that  described  in  the  last  paragraph. 
The  hydrogen-atom  united  to  the  remaining  bond  will 
now  become  by  induction  electro  -positive,  thus  : 


and,  consequently,  if  we  bring  riear  the  molecule  a  rad- 
ical like  K,  which,  by  its  nature,  has  a  highly  electro- 
positive polarity,  the  molecule  will  drop  the  hydrogen 
and  take  in  its  place  the  potassium  atom.  It  is  the 
preference  for  a  negative  radical  in  place  of  its  hydro- 
gen-atom which  makes  the  first  molecule  alkaline,  while 
it  is  a  similar  preference  for  a  positive  radical  which 
renders  the  second  molecule  acid  ;  and  these  preferences, 
as  we  now  see,  are  manifestations  of  energy  similar  to 
those  with  which  we  are  familiar  in  that  well-known 
mode  of  polarity  called  magnetism. 

Let  me  not,  however,  be  understood  to  imply  that 
the  analogy  here  presented  is  perfect,  or  that  it  can  be 
followed  out  into  details  ;  for  this  is  far  from  being 
true.  If  chemism  is,  as  it  seems  to  be,  a  mode  of 
polar  action,  it  manifests  characteristics  which  find 
their  parallel  in  electrical  rather  than  in  magnetic  phe- 
nomena. One  instance  of  the  failure  of  the  analogy 
I  have  drawn  we  meet  at  once  —  and  you  have  probably 


300  ELECTRO-CHEMICAL   THEORY. 

already  detected  it — in  that  important  but  small  class 
of  acids  of  which  hydrochloric  acid  is  the  type.  The 
molecules  of  these  compounds  consist  of  a  single  hy- 
drogen-atom united  to  a  highly-negative  radical,  and 
this  hydrogen -a  torn  has  the  same  susceptibility  of  re- 
placement by  positive  radicals,  which  is  the  essential 
characteristic  of  the  acid  hydrates  we  have  been  study- 
ing. These  molecules  contain  no  oxygen,  and  how, 
you  may  ask,  can  the  theory  of  the  constitution  of  acids 
and  alkalies  we  have  been  expounding  apply  to  them  ? 
The  only  answer  we  can  give  is,  that  they  appear  to 
present  a  simpler  type  of  polarity,  to  which,  though 
unlike  magnetism,  we  have  a  parallel  in  the  phenomena 
of  electricity. 

Take,  for  instance,  the  molecule  of  hydrochloric 
acid,  HC1,  the  best  example  of  its  class.  In  this  the 
chlorine-atom  seems  to  have  a  single  pole,  which  is 
strongly  negative,  and  by  its  influence  there  appears 
to  be  induced  an  opposite  pole,  also  single,  in  the  atom 
of  hydrogen.  If,  now,  we  bring  near  to  this  binary 
group  an  atom  like  Na,  which  either  has  by  itself,  or 
is  capable  of  acquiring  by  induction,  a  higher  degree 
of  positive  polarity  than  H,  then  the  chlorine  pole  drops 
the  H  and  takes  the  Na. 

In  the  polar  condition  thus  developed,  the  two  op- 
posite poles  are  on  different  atoms,  and  not  only  are 
the  two  atoms  separable,  but  the  positive  or  negative 
virtue  appears  to  be  inherent  in  the  atom,  and  is  trans- 
ferred with  it.  A  magnetic  pole,  on  the  contrary,  is 
always  associated  with  its  opposite  on  the  same  mass  of 
metal,  and,  if  the  mass  is  divided,  two  poles  are  found 
on  each  of  the  fragments,  and  so  on  indefinitely,  however 
far  the  division  may  be  carried.  In  the  phenomena  of 
statical  electricity,  however,  we  have  a  well-defined 


ELECTRICAL   ANALOGIES.  301 

condition  of  polarity,  to  which  the  example  of  chemism 
we  have  been  just  discussing  appears  to  be  closely  al- 
lied. If  a  pith-ball,  electrified  positively  (or  vitreous- 
ly),  is  brought  near  a  similar  ball  electrified  negatively 
(or  resinously),  they  attract  each  other,  and  the  one  be- 
comes the  pole  of  the  other.  If,  now,  the  two  are  sep- 
arated, each  carries  with  it  its  electrical  charge,  and 
the  peculiar  virtue  it  has  in  consequence  of  that  charge. 
But,  though  the  two  poles  may  thus  be  separated,  and 
cease  to  have  any  relation  to  each  other,  yet  they  do 
not  become  isolated  in  any  proper  sense  of  that  term, 
for  each  of  the  electrified  bodies  draws,  by  induction, 
an  electrical  charge,  opposite  to  its  own,  to  the  extrem- 
ity of  the  nearest  conductor,  and  this  charge  becomes 
a  new  pole.  An  isolated  pole  is,  in  fact,  a  contradic- 
tion of  terms.  Polarity  implies  an  opposition  of  rela- 
tions, which  involves  two  poles,  and  electrical  polarity 
differs  from  magnetic  polarity  chiefly  in  the  circum- 
stance that  the  two  poles  are  separate  bodies.  The  mag- 
netic poles  are  the  ends  of  a  polarized  bar  of  iron,  while 
the  electrical  poles  are  the  boundaries  of  a  mass  of  po- 
larized dielectric,  usually  air,  which  intervenes  between 
the  oppositely  electrified  bodies  ;  and  every  charge  of 
electricity  is  just  as  closely  associated  with  an  opposite 
charge  resting  on  some  conductor  beyond  the  insulating 
dielectric,  as  one  magnetic  pole  accompanies  the  other. 
Now,  it  is  worthy  of  remark  that  this  indissoluble 
association  of  opposite  poles,  which  we  must  expect  to 
find  in  chemical  phenomena,  if  chemism  is,  as  we  sup- 
pose, a  polar  force,  is  actually  manifested  in  a  striking 
class  of  facts.  The  univalent  atoms  which,  like  those 
of  chlorine  or  sodium,  act  as  single  poles,  are  never 
found  isolated,  but  are  always  associated  in  a  mole- 
cule with  at  least  one  other  atom  which  forms  the  op 


302  ELECTRO-CHEMICAL   THEORY. 

posite  pole  of  the  molecular  system,  and,  although  the 
two  poles  of  a  molecule  like  HC1  can  be  readily  sepa- 
rated, the  atoms  do  not  remain  isolated,  but  immedi- 
ately form  new  associations,  as  in  this  very  case,  where 
the  atoms  of  hydrogen  pair  off  into  molecules  of  hy- 
drogen gas  (H-H),  and  those  of  chlorine  into  molecules 
of  chlorine  gas  (C1-C1),  which  are  polar  systems  similar 
to  those  destroyed.  On  the  other  hand,  bivalent  atoms, 
like  those  of  mercury  or  zinc,  which  have  two  poles,  and 
may,  therefore,  constitute  a  complete  polar  system,  each 
by  itself,  are  sometimes  found  isolated,  and  form  that 
class  of  molecules,  previously  described,  in  which  the 
molecules  consist  of  single  atoms.  The  phenomena  of 
quanti valence,  also,  which  are  such  a  characteristic  feat- 
ure of  what  we  may  now  call  chemical  polarity,  have 
their  parallel  in  the  phenomena  of  multiple  poles,  so 
familiar  in  magnetism,  and  may  be  caused  by  the  same 
polar  force  acting  through  atoms  of  different  shapes, 
and  susceptibility  to  its  influence  ;  and  the  fact  already 
referred  to,  that,  in  the  variations  of  quantivalence,  two 
bonds  always  appear  or  disappear  at  a  time,  is  a  strong 
confirmation  of  this  theory  ;  for,  as  has  been  said,  one 
pole  implies  an  opposite  of  equal  strength,  and  the  two 
must  stand  or  fall  together.  It  would  be  a  further  con- 
sequence of  the  theory  that,  although  atoms  of  any 
even  degree  of  quantivalence  (artiads)  might  become 
isolated  in  molecules,  those  of  an  uneven  degree  (peris- 
sads)  could  not ;  and  this  also  we  find  to  be  true  so  far 
ns  observation  extends  ;  but  the  number  of  elementary 
substances  whose  molecular  weight  has  been  directly 
determined  is  comparatively  small,  and  those  whose 
molecules  are  known  to  consist  of  single  atoms,  al- 
though all  artiads,  are  only  bivalent. 

rn  ing  now  for  a  moment  to  the  simple  type  of 


ELECTRICAL   ANALOGIES.  303 

polarity  presented  by  the  molecule  H-C1,  let  me  call 
your  attention  to  the  fact  that  the  polarity  of  the  ordi- 
nary acid  hydrates  is  but  a  modified  form  of  the  sim- 
pler type,  and  this  will  be  obvious  on  comparing  the 
symbol  of  hydrochloric  acid  with  that  of  hypochlorous 
acid,  from  which  it  differs  only  by  an  atom  of  oxygen : 

H-C1  H-~O-C1. 

Hydrochloric  Acid.  Hypochlorous  Acid. 

You  will  notice  that  the  atoms  H  and  Cl  are  the  poles 
of  both  systems,  and  that  the  oxygen-atom  in  the  last 
is  analogous  to  an  armature  between  two  magnetic 
poles,  or,  perhaps,  more  closely  to  a  prime  conductor 
between  two  oppositely-electrified  balls  : 

0     (Z        H)     © 
H  O  Cl, 

Hypochlorous  acid  illustrates  this  relation  more  strik- 
ingly than  nitric  acid,  our  previous  example  of  this 
class  of  compounds,  but  it  is  not  nearly  so  stable  a  sub- 
stance, and  has  never  been  obtained  in  a  pure  condi- 
tion. Nitric  acid  differs  from  hypochlorous  acid  in  con- 
taining a  compound  in  place  of  a  simple  radical — 

H-~0-Cl.  H-~0-(N02). 

Hypochlorous  Acid.  Nitric  Acid. 

and  the  presence  of  compound  radicals,  often  very  com- 
plex, in  the  molecules  of  all  the  well-marked  acids,  ne- 
cessarily increases  the  difficulty  of  interpreting  their  mo- 
lecular structure,  since  the  symbols  may  frequently  be 
grouped  in  several  ways  without  violating  the  principles 
of  quanti  valence.  Our  theory  of  the  molecular  struct- 
ure of  acid  hydrates  cannot,  therefore,  afford  to  waive 
the  important  evidence  in  its  favor  which  has  been  ob- 
tained from  recent  investigations,  and,  as  I  am  anxious 
to  establish  it  on  such  a  firm  foundation  that  it  may  be 


304  ELECTRO-CHEMICAL   THEORY. 

taken  as  a  basis  in  our  further  investigations  of  mo- 
lecular structure,  I  must  ask  you  to  listen  patiently  to 
the  few  additional  points  I  have  to  present. 

The  element  carbon  forms,  with  oxygen,  besides  the 
compound  carbonic  dioxide,  which  we  have  already 
studied,  a  second  compound,  called  carbonic  oxide, 
which  has  the  symbol  C=O.  In  this  molecule  two  of 
the  bonds  of  the  carbon -atom  are  unemployed,  or, 
rather,  neutralized  by  their  mutual  attraction.  Hence, 
these  molecules  are  very  much  in  the  same  condition 
as  the  atoms  of  mercury  or  zinc,  when  acting  as  mole- 
cules, and,  like  them,  the  molecule  CO  can  enter  into 
direct  combination,  as  a  bivalent  radical.  Striking  in- 
stances of  such  combination  are  the  formation  of  phos- 
gene gas  by  the  direct  union  of  carbonic  oxide  with 
chlorine  gas,  under  the  influence  of  sunlight,  and  the 
burning  of  carbonic  oxide,  when  the  same  molecules 
unite  with  an  additional  atom  of  oxygen  to  form  car- 
bonic dioxide  : 

O 

CO      +      01 -Cl      =      C0012,    or    C1-C-C1; 

Carbonic  Oxide.      Chlorine  Gas.          Phosgene  Gas. 

20O       +       O  =  O       =       2CO2    or    O  =  C  =  O. 

Carbonic  Oxide.        Oxygen  Gas.       Carbonic  Dioxide. 

Now,  if  potassic  hydrate,  K-O-H,  is  gently  heated 
in  an  atmosphere  of  carbonic  oxide,  a  slow  but  regular 
absorption  of  the  gas  takes  place,  and  the  potassium 
salt  of  a  well-known  acid,  called  formic  acid,  is  the  re- 
sult, and,  from  a  mixture  of  this  salt  with  sulphuric  acid, 
we  can  readily  distill  off  the  acid  itself.  Formic  acid 
being  volatile,  we  can  determine  with  certainty  its 
molecular  weight,  and,  since  an  accurate  analysis  is  also 
possible,  there  is  no  doubt  whatever  that  the  symbol 
H2O2C  expresses  the  exact  composition  of  its  molecule. 
But  how  are  these  atoms  arranged  ?  As  data  for  solv* 


SYNTHESIS   OF   FORMIC   ACID.  305 

ing  this  problem,  we  have,  in  the  first  place,  the  known 
quantivalence  of  the  several  atoms,  and,  in  the  second 
place,  a  knowledge  of  the  fact,  acquired  in  studying  the 
phenomena  of  combustion,  that,  if,  in  the  reaction  by 
which  formic  acid  was  produced,  the  two  atoms  of 
the  radical  CO  had  been  parted,  an  enormous  absorp- 
tion of  heat  must  have  attended  the  chemical  change. 
But  no  such  thermal  effect,  nor  any  of  the  phenom- 
ena, which  would  naturally  accompany  it,  have  been 
noticed,  and  we  therefore  feel  justified  in  concluding 
that  the  radical  CO  exists  as  such  in  formic  acid,  as 
the  direct  absorption  of  the  gas  by  caustic  potash  would 
seem  to  indicate.  The  only  question  that  remains  is, 
how  the  other  atoms  are  grouped  around  this  radical, 
and  the  quantivalence  of  the  atoms  permits  but  one 
mode  of  grouping,  as  follows  : 

O 
H-O-C-H. 

In  this  molecule  there  are  two  atoms  of  hydrogen, 
one  united  directly  to  the  carbon-nucleus,  the  other  also 
united  to  the  same  radical,  but  only  indirectly  through 
the  atom  of  oxygen  which  intervenes.  'Now,  are  both 
of  these  hydrogen-atoms  equally  susceptible  of  replace- 
ment? We  find  not.  If  we  neutralize  the  acid  by 
potassic  hydrate,  we  obtain  the  same  potassium  salt 
which  was  formed  by  the  direct  union  of  the  alkali 
with  carbonic  oxide,  and  analysis  shows  that  this  salt 
contains  just  one-half  as  much  hydrogen  as  the  acid 
from  which  it  was  formed,  and,  by  no  metathetical  re- 
action Avhatever  can  we  succeed  in  replacing  the  re- 
maining atom. 

Evidently,  then,  the  two  atoms  stand  in  very  differ- 
ent relations  to  the  molecule ;  but  which  was  the  one 
replaced  ?  As  to  this  point,  we  have  the  most  conclu- 


306  ELECTRO-CHEMICAL   THEORY. 

sive  and  abundant  evidence.  "We  need  call,  however, 
but  a  single  class  of  witnesses.  Formic  acid  is  the  first 
of  a  series  of  volatile  acids,  and  the  molecules  of  the  suc- 
cessive compounds  which  form  the  steps  of  this  series 
differ  from  each  other  by  the  common  difference  CH2. 
The  second  member  of  the  series  is  acetic  acid,  which, 
in  a  diluted  condition,  is  used  as  a  condiment  with  our 
food  under  the  name  of  vinegar.  The  composition  of 
pure  acetic  acid  is  represented  by  the  symbol  H4O2C2, 
and  the  molecule  of  this  acid,  therefore,  contains  four 
atoms  of  hydrogen.  But  of  these  only  one  is  replaceable 
— as  in  formic  acid — and  the  same  is  true  of  all  the  acids 
of  this  class,  although  the  molecules  of  the  last  member 
of  the  series  contains  no  less  than  sixty  hydrogen-atoms. 
Moreover,  acetic  acid — like  formic  acid — contains  two 
atoms  of  oxygen,  and  two  corresponding  atoms — and 
only  two — appear  in  the  molecules  of  all  the  other 
members  of  the  same  series.  Add  now  the  further 
fact,  which  will  be  illustrated  more  fully  hereafter,  that 
several  of  the  compounds  in  the  series  have  been  pre- 
pared from  formic  acid  by  processes  which  show  that, 

if  the  radical 

O 

ii 

H-O-C- 

exists  in  the  molecule  of  this,  the  first  member  of  the 
series,  it  must  also  form  a  part  of  the  molecules  of  all 
the  other  members,  and  you  will  be  prepared,  I  think, 
to  answer  the  question  proposed  above.  The  facts  stated 
may  be  almost  said  to  prove  that  in  all  these  molecules 
one,  and  only  one,  atom  of  hydrogen  is  united  to  the 
radical  by  an  atom  of  oxygen,  and  this  must  be  the  sin- 
gle atom  which  in  all  these  compounds  is  susceptible  of 
replacement.  We  may,  therefore,  write  the  symbol  of 
formic  acid- 


SYNTHESIS  OF  FORMIC  ACID.  307 

and  regard  the  molecule  as  having  a  polar  condition 
like  that  we  attributed  to  the  molecule  of  nitric  acid. 

Here,  then,  is  a  well-marked  acid,  in  regard  to  the 
structure  of  whose  molecule  there  can  be  no  reasonable 
doubt,  and  the  conclusion  we  have  reached  in  regard 
to  it  harmonizes  completely  with  that  we  had  pre- 
viously formed  in  regard  to  the  structure  of  the  mole- 
cule of  nitric  acid  on  wholly  different  grounds.  Such 
a  concurrence  of  testimony  gives  us  great  confidence 
in  the  theory  we  have  advanced  in  regard  to  the  con- 
stitution of  this  class  of  substances,  and  we  may  cer- 
tainly accept  it  as  a  trustworthy  guide  in  the  further 
prosecution  of  our  study. 

It  will  not,  of  course,  be  for  a  moment  inferred 
that  we  regard  the  argument  now  concluded  as  demon- 
strative. We  have  been  advocating  what  we  have 
expressly  called  a  theory,  and  all  we  claim  is  that  the 
evidence  advanced  is  sufficiently  conclusive  to  render 
the  theory  credible,  and  that  the  theory  is  of  great  val- 
ue, both  by  giving  us  a  more  comprehensive  grasp  of 
the  facts  with  which  we  have  to  deal,  and  by  helping 
us  to  associate  the  supersensuous  phases  of  molecular 
action  with  the  visible  phenomena  of  magnetism  and 
electricity. 

Having  then  stated,  as  fully  as  the  circumstances 
will  permit,  the  evidence  on  which  our  theory  of  the 
constitution  of  acids  and  alkalies  rests,  in  the  case  of 
a  few  of  the  simpler  of  these  compounds,  I  must,  as 
regards  the  molecular  structure  of  the  more  complex 
compounds  of  the  same  type,  content  myself  with  mere- 
ly stating  results,  only  premising  that  the  conclusions 
rest  on  evidence  similar  to  that  already  adduced. 

Beginning  with  the  series  of  volatile  acids,  of  which 
formic  and  acetic  acids  are  members,  let  me  first  call 


308  ELECTRO-CHEMICAL  THEORY. 

your  attention  to  the  following  symbols,  which,  as  we 
believe,  represent  the  molecular  structure  of  these 

bodies : 

O 

Formic  acid H  -  O  -  (C  -  H) 

O    H 

Acetic  acid H-O-(C-C-H) 

H 

O    H    H 

Propionic  acid H-O-(C-C-C-H) 

H    H 

O    H    H    H 

I!          I          I          I 

Normal  butyric  acid H-O-(C-C-C-O-H) 

i      i      i 
H    H    H 

O    H    H    H    H 

Normal  valeric  acid H-0-  (6  -  6-6  -6-6-H) 

i      i      i      i 
H    H    H    H 

All  the  above  compounds  have  been  thoroughly  inves- 
tigated, and  all  the  symbols  given  above  rest  on  as 
good  evidence  as  the  first.  All  these  compounds  have 
the  same  general  structure,  and  the  same  system  of 
polarity,  as  the  simpler  hydrates,  and  they  may  be  re- 
garded as  derived  from  formic  acid  by  successive  sub- 
stitutions of 

H 

-C-H 

i 
H 

for  the  final  hydrogen-atom  of  the  negative  radical. 
Lastly,  notice  the  binary  group,  H-O-,  which  plays  such 
an  important  part  in  these  and  all  similar  molecules. 
This  group  of  atoms,  or  radicals,  has  been  named  hy- 


DEFINITION  OF  HYDRATES.  309 

droxyl,  and,  for  the  future,  we  shall  find  it  convenient 
to  employ  this  term. 

In  all  the  examples  thus  far  cited,  in  illustration  of 
our  theory  of  the  molecular  structure  of  acid  and  alka- 
line hydrates,  the  molecule  has  contained  but  one  hy- 
droxyl  (HO)  group,  and  therefore  but  one  replaceable 
hydrogen-atom.  Such  hydrates  are  said  to  be  mon- 
atomic.  While,  however,  the  univalent  radicals,  which 
these  compounds  all  contain,  can  only  bind  one 
hydroxyl  group,  a  bivalent  radical  may  be  associated 
with  two  such  groups,  a  trivalent  radical  with  three_, 
and  so  on.  In  the  resulting  compound  there  will  be  as 
many  replaceable  atoms  of  hydrogen  as  there  are  hy- 
droxyl groups  united  to  the  radical,  and  the  number  of 
these  replaceable  atoms  measures  what  is  called  the 
atomicity  of  the  compound.  We  are  now  prepared  to 
define  also  the  term  hydrate,  that  we  have  so  frequently 
used  in  this  lecture  to  designate  the  class  of  compounds 
to  which  all  the  alkalies  and  most  of  the  acids  belong. 
A  hydrate  is,  simply,  a  compound  of  hydroxyl,  and  is 
monatomic,  diatomic,  triatomic,  etc.,  according  as  it  con- 
tains one,  two,  three,  or  more  hydroxyl  groups.  Let  me 
illustrate  this  important  principle  by  a  few  examples 
of  hydrates  of  multivalent  radicals,  beginning  with 
those  in  which  the  radical  is  bivalent. 

At  the  boiling-point,  metallic  magnesium  slowly  de- 
composes water,  liberating  hydrogen  gas  — 


2H2O       +       Mg      =       M^02H2        +        H-H 

Water.  Magnesium.        Magnesic  Hydrate.        Hydrogen  Gas. 

In  this  reaction  the  bivalent  atom  of  magnesium 
binds  together  two  molecules  of  water  to  form  a  mole- 
cule of  magnesic  hydrate,  whose  structure  may  be  rep- 

resented : 

H-0-M^-O-H. 

Magnesic  Hydrate. 


310  ELECTRO-CHEMICAL  THEORY. 

The  molecule  of  common  slacked  lime,  calcic  hydrate, 
has  a  similar  structure  : 

H-0-Ca-O-H. 

Calcic  Hydrate. 

These  two  hydrates  are  both  alkaline,  but  there  are 
corresponding  acid  hydrates,  among  which  are  num- 
bered the  two  very  important  chemical  agents  called 
sulphuric  and  oxalic  acids,  whose  molecules  are  sup- 
posed to  have  the  structure  indicated  by  our  diagrams  : 

O  O   O 

H-0-S-O-H  H-0-C-C-O-H 

II  Oxalic  Acid. 

O 

Sulphuric  Acid. 

Compounds  like  the  last  four  are  said  to  be  diatomic ; 
for  there  are  in  each  case  two  hydroxyl  groups,  and 
therefore  two  easily-replaceable  atoms  of  hydrogen,  and 
this  is  shown,  in  the  case  of  the  acids,  by  the  fact  that, 
when  wholly  or  one-half  neutralized  with  caustic  soda 
or  potash,  they  give  two  different  salts,  in  one  of  which 
the  whole,  and  in  the  other  only  one-half,  of  the  hy- 
drogen of  the  acid  is  replaced.  Thus,  we  have — 

O  O 

H-0-S-O-Na  Na-0-S-O-Na 

ii  ii 

O  O 

Hydrosodic  Sulphate.  Disodic  Sulphate. 

So  also — 

O   O  O    O 

H-0-C-C-O-K  K-0-C-C-O-K 

Hydropotassic  Oxalate.  Dipotassic  Oxalate. 

If,  however,  we  neutralize  these  dibasic  acids  with 
inagnesic  or  calcic  hydrates,  we  can  obtain  but  one 
product,  because  the  bivalent  atoms  Mg  and  Ca  replace 
the  two  hydrogen  atoms  at  once.  The  salts  thus  ob- 
tained have  the  symbols : 


INSTABILITY  OF  COMPLEX  HYDRATES.  311 


MS\O/b^O  O-C  =  O 

Magnesic  Sulphate.  Calcic  Oxalate. 

It  may,  perhaps,  avoid  some  confusion  to  repeat 
here  the  remark  already  made,  that  the  position  or 
grouping  of  the  symbols  on  the  diagram  is  wholly  ar- 
bitrary beyond  the  relations  which  the  dashes  indicate. 

Pass  next  to  hydrates  which  contain  three  hydroxyl 
groups,  and  are,  therefore,  said  to  be  triatomic.  Of 
these  we  shall  only  cite  two  examples  : 

H  H 

i  i 

O  O 

H-O-B-O-H  H-O-P-O-H 

Boric  Acid.  II 

O 
Phosphoric  Acid. 

The  triatomic  character  of  phosphoric  acid  is  shown  by 
the  fact  that  it  can  be  neutralized  by  caustic  soda  in 
three  successive  stages,  and  gives  three  compounds,  one 
of  which  contains  no  hydrogen,  and  the  others  respec- 
tively one-third  and  two-thirds  as  much  as  in  the  corre- 
sponding quantity  of  the  acid.  The  names  and  symbols 
of  these  salts  are  as  follows  : 

Na3=03EPO  H,tfa2=CMPO  H^Na^PO 

Trisodic  Phosphate.         Hydrodisodic  Phosphate.         Dihydrosodic  Phosphate. 

This  abbreviated  form  of  notation  can  be  easily  under- 
stood, and  requires  no  further  explanation.  It  saves 
space  in  printing,  and  gives  all  the  data  required  for 
constructing  the  graphic  symbols. 

Of  hydrates  containing  four  hydroxyl  groups,  there- 
fore, tetratomic,  the  most  familiar  is  silicic  hydrate  — 

H-(Xq./0-H 
H-O/bl\O-H 

but  this  substance  is  very  unstable,  and  hitherto  it  has 


312  ELECTRO-CHEMICAL  THEORY. 

been  impossible  to  prepare  it  of  constant  composition. 
The  instability  is  due  to  a  cause  which  is  inherent 
in  many  of  the  more  complex  molecular  structures. 
Wherever  there  is  a  tendency  in  the  atoms  to  group 
themselves,  so  as  to  better  satisfy  their  mutual  affini- 
ties, a  slight  cause  is  sufficient  to  destroy  the  balance 
of  forces  on  which  the  existence  of  the  molecule  de- 
pends, and  the  structure  breaks  up  into  simpler  parts. 
The  explosion  of  nitro-glycerine  was  a  conspicuous  ex- 
ample of  this  principle,  and  we  have,  in  these  complex 
hydrates,  another  illustration  of  the  same.  It  is  evi- 
dent, from  the  very  great  amount  of  heat  evolved  in 
the  direct  union  of  oxygen  and  hydrogen  gases,  that 
the  molecules  of  water  are  in  a  condition  of  great  sta- 
bility, and  the  hydrogen  and  oxygen  atoms,  which  are 
associated  in  such  numbers  in  the  molecules  of  the 
more  complex  hydrates,  are  constantly  tending  to  this 
condition  of  more  stable  equilibrium.  Indeed,  these 
compounds  give  off  water  so  readily,  either  spontane- 
ously or  at  the  slightest  elevation  of  temperature,  that 
they  were  formerly  supposed  to  contain  water,  as  such, 
and  hence  the  name  hydrates  (from  vSwp,  water),  which 
has  been  retained  in  our  modern  nomenclature,  al- 
though with  a  modified  meaning. 

Since  the  number  of  oxygen  and  hydrogen  atoms 
in  the  several  hydroxyl  groups  united  to  the  radical  of 
a  hydrate  must  necessarily  be  the  same,  it  follows  that 
the  formation  of  every  molecule  of  water  must  be  at- 
tended with  the  liberation  of  an  atom  of  oxygen,  and, 
when  a  hydrate  breaks  up,  these  atoms  frequently  unite 
with  the  radical  to  form  compound  radicals  of  lower 
quantivalence.  Thus  we  have  formed  from  the  normal 
silicic  hydrate,  by  the  elimination  of  successive  mole- 
cules of  water,  the  following  products  : 


SILICIC  HYDRATES.  313 


H 

H 

i 

i 

0 

O 

i 

i 

H- 

O-Si-O-H 

Si  =  O 

i 

i 

O 

0 

i 

i 

H 

H 

Normal  Hydrate. 

First  Anhydride.1 

Second  Anhydride. 

The  atoms  of  oxygen  liberated  as  just  described 
may  also  bind  together  several  atoms  of  silicon,  and 
thus  give  rise  to  still  more  complex  groups,  such  as  — 

H          H  H          H          H 

ii  it! 

O  O  O          O          O 

H-O-Si-O-Si-O-H  H-O-Si-O-Si-O-Si-O-H 

ii  lit 

O  O  O          O          O 

II  III 

H          H  H          H         H 

H          H  H  H 

ii  i  i 

00  O  O 

Si  \  Q  )  Si  Si  \  Q  /  Si  N  Q  /  Si 

O          O  O  O 

II  I  I 

H          H  H  H 

These  compounds  may  be  regarded  as  formed  by  the 
coalescing  of  two  or  more  molecules  of  the  normal  hy- 
drate, and  the  elimination  from  these  combined  mole- 
cules of  successive  molecules  of  water  as  before.  The 
following  table  will  illustrate  what  is  meant  : 

H4O4Si  2(H4O4Si)  3(H4   O4  Si) 

H2  O2  SiO  H6  O8  Si2O  H10  O10  Si3O 

SiOa  H4  O4  Si2O2  H8  O8  Si3O2 

H2O2Si2O3  H6  O6  Si3O3 

2SiOa  H4   04   Si3O4 

H2   O2   Si3O5 
3SiO2 

1  A  compound  derived  from  a  hydrate  by  the  elimination  of  water  is 
called  an  anhydride. 
'22 


314  ELECTRO-CHEMICAL  THEORY. 

The  table  might  be  extended  indefinitely.  It  is  true 
that  not  every  member  of  these  series  is  even  theoreti- 
cally a  possible  compound ;  but,  by  attempting  to  write 
the  symbols  in  the  more  graphic  form,  those  cases  in 
which  the  atoms  cannot  be  grouped  in  a  single  mole- 
cule will  be  readily  distinguished. 

We  have  in  this  glass  a  solution  of  sodic  silicate, 
which  is  commonly  called  soluble  glass.  On  adding  to 
the  solution  some  muriatic  acid,  you  notice  that  there 
is  at  once  formed  a  white,  bulky,  gelatinous  mass. 
This  is  supposed  to  be  the  normal  silicic  hydrate,  but, 
when  we  attempt  to  wash  and  dry  the  substance  for 
the  purpose  of  analysis,  it  begins  to  lose  water,  and 
we  have  found  it  impossible  to  arrest  the  change  at  any 
definite  point.  In  the  process  of  drying,  the  various 
hydrates,  whose  symbols  we  have  given,  are  probably 
produced,  but  only  as  passing  phases  of  the  dehydra- 
tion, and  these  symbols  would  be  wholly  ideal  were  it 
not  that,  on  replacing  the  hydrogen-atoms  by  metallic 
radicals,  we  obtain  products  of  great  stability.  The 
compounds  to  which  I  refer  are  the  mineral  silicates 
that  form  so  large  a  part  of  the  minerals  and  rocks  of 
the  globe.  The  two  following  well-known,  although 
not  abundant,  minerals  correspond,  for  example,  to  the 
normal  hydrate  and  its  first  anhydride  respectively  : 

Mg(g)Si(g)Mg  Ca(°)Si  =  0 

Magnesia  Chrysolite.  Wollastonite. 

and  the  symbols  show  that  the  molecular  structures  we 
have  described  above  are  realized  in  these  natural  prod- 
ucts if  not  in  the  hydrates.  The  molecular  structure 
of  some  of  our  most  common  minerals,  such  as  feldspar 
and  garnet,  corresponds  to  that  of  some  of  the  most 
complex  hydrates,  with  radicals  consisting  of  several 


MINERAL  SILICATES.  315 

silicon-atoms  ;  but,  we  shall  understand  better  the  man- 
ner in  which  these  highly-complex  molecules  are  built 
up,  after  we  have  become  acquainted  with  a  remarkable 
hexatomic  hydrate,  whose  well-marked  sexivalent  radi- 
cal plays  a  very  important  part  in  their  structure. 

No  definite  pentatomic  hydrate  is  known,  but  of 
hexatomic  hydrates  there  are  several  noteworthy  ex- 
amples. The  one  referred  to  in  the  last  paragraph  is 
the  hydrate  of  aluminum.  The  normal  hydrate  of  this 
element,  and  the  several  anhydrides  which  may  be 
formed  from  it  by  the  elimination  of  successive  mole- 
cules of  water,  are  all  well-defined  mineral  substances. 
The  following  table  shows  the  relations  of  these  com- 
pounds to  each  other,  and  also  to  certain  other  mineral 
substances  in  which  the  hydrogen-atoms  have  been  re- 
placed : 

AVCVHc  O=A12=04=H4  O2iAl2=02=H2  O3TIA12 

Gibbsite.  Beauxite.  Diaspore.  Corundum. 

O=Al2=CMSi  O2=A12=O2=G 

Andalusite.  Chrysoberyl. 

It  would  be  interesting  to  represent  in  a  graphic  form 
these  molecules,  but  I  can  leave  this  to  your  own  study, 
and  close  my  illustrations  of  the  subject  with  two  or 
three  examples  of  *he  very  highly-complex  molecular 
structures  which  the  salts  of  aluminum  present,  and  in 
which  the  mode  of  atomic  grouping  is  less  obvious  : 


H   H      O  Q    \)          O       H   H 

\   /  II  II  I!  \   / 

JST-O-S-O-A1-A1-O-S-0-N 

/  \          H  ii  H          /  \ 

HHO  OO          OHH 

\  / 
0  =  8  =  0 

Ammonia  Alum  (dried.) 


316  ELECTRO-CHEMICAL  THEORY. 


00  00 

££ 

Garnet  (Lime  Alumina). 

O 

\  /      \ 

Si-O-Si 

/    \         /    \ 
O        O       O        O 

K-O-Si-O-Al-Al-O-Si-O-K 

\  /      \  / 

O        O       O        O 

\    /         \    / 

Si-O-Si 

\      / 

O 

Feldspar  (Orthoclase). 

In  arranging  these  symbols  for  our  diagrams,  we  natu- 
rally seek  a  symmetrical  disposition ;  but  it  must  not  be 
forgotten  that  every  thing  beyond  the  number  of  atomic 
bonds,  and  the  relative  position  which  the  dashes  indi- 
cate, is  purely  arbitrary. 

I  have  dwelt  at  this  length  on  the  theory  of  the 
acid  and  alkaline  hydrates,  because  it  is  just  here  that 
the  distinction  between  the  new-s&hool  and  the  old- 
school  chemistry  chiefly  appears.  The  dualistic  theory, 
which  originated  with  Lavoisier,  and  was  extended  and 
illustrated  by  Berzelius,  was  based  on  the  very  class 
of  facts  we  have  been  studying  in  the  two  preceding 
lectures  of  this  course.  At  the  time  of  Berzelius,  the 
elements,  the  acids,  the  alkalies,  or  bases,  and  the 
large  class  of  compounds  called  salts,  made  up  very 
nearly  the  whole  of  chemistry,  and,  of  the  facts  then 
known,  the  dualistic  theory  gave  a  satisfactory  explana- 
tion. It  was  the  natural  outgrowth  of  the  discovery 


THE  DUALISTIC  THEORY.  317 

of  oxygen  gas,  that  universally-diffused  element  with 
which  all  other  elementary  substances  combine,  and  of 
whose  compounds  almost  the  whole  of  terrestrial  Nature 
consists.  Lavoisier  inferred  that  oxygen  must  be  the 
chemical  centre  in  the  scheme  of  Nature,  and  he  there- 
fore made  its  compounds  the  basis  of  a  new  classifica- 
tion, which,  subsequently,  Berzelius  greatly  systema- 
tized and  improved.  In  this  classification  the  com- 
pounds of  the  elements  with  oxygen  were  divided  into 
two  classes  :  Those  which,  when  dissolved  in  water — 
combined  with  it  we  should  now  say — gave  an  acid  re- 
action, were  called  acids ;  while  those  which,  under  the 
same  circumstances,  gave  an  alkaline  reaction,  were 
called  bases.  It  was  known  then,  as  well  as  now,  that 
these  reactions  could  not  be  obtained  without  the  pres- 
ence of  water,  and  that  the  larger  part  of  the  oxides, 
being  insoluble  in  water,  do  not  give  the  reactions  at 
all ;  but,  then  it  wns  supposed  that  the  water  acted 
only  through  virtue  of  its  solvent  power,  that  some 
other  solvent  would  do  as  well,  and  that  the  insoluble 
oxides  would  give  the  same  reactions  if  only  an  appro- 
priate solvent  could  be  found.  Hence,  these  insoluble 
oxides  were  classed  with  the  acids  or  bases,  according 
as  they  combined  most  readily  with  bases  or  acids  re- 
spectively. The  insoluble  SiO2  combined  with  soda, 
like  the  soluble  SO3,  and  hence  was  classed  with  it  as 
an  acid.  So  the  insoluble  FeO  combined  with  sulphu- 
ric acid,  like  the  soluble  CaO,  and  hence  was  classed 
with  the  last  as  a  base.  Again,  the  neutralizing  of  an 
acid  by  an  alkali  had  all  the  appearance  of  direct  combi- 
nation, and,  in  all  these  processes,  the  acid  oxide  was  as- 
sumed to  unite  with  the  metallic,  or  basic,  oxide  to  form 
what  was  called  a  salt.  The  presence  of  the  water,  and 
the  fact  that  it  facilitated  the  chemical  change,  were  not 


318  ELECTRO-CHEMICAL  THEORY. 

ignored,  but,  as  before,  it  was  supposed  to  act  in  virtue 
of  its  solvent  power,  and  a  sufficient  number  of  cases 
were  known  where  the  same  compounds  could  be  ob- 
tained with  and  without  the  aid  of  water  to  render  this 
opinion  not  improbable.  Take  a  single  example  :  Phos- 
phate of  lime  may  be  made  in  two  ways  :  first,  by  add- 
ing to  a  solution  of  lime  in  water  a  solution  of  phos- 
phoric acid : 

(3Ca=Oa=H2  +  2H3=O3=PO  +  Aq.)  = 

Ca3vl(V(PO)2  +  (6H-0-H  +  Aq.). 

Secondly,  by  uniting  lime,  the  oxide  of  the  metal  cal- 
cium, directly  to  P2O5,  the  oxide  obtained  by  burning 
phosphorus  (page  212) : 

3CaO  +  P206  =  3CaO,P2O3,      or      Ca3Ti06vl(PO)3. 

In  the  last  reaction  there  is  no  water  present,  and  the 
first  reaction  was  formerly  supposed  to  be  a  case  of 
similar  direct  union  between  CaO  and  P2O5,  the  only 
difference  being  that  the  two  oxides  were  in  solution : 

3(CaO,H2O)  +  3H20,P206  =  3CaO,P2O5  +  6H2O. 

Accordingly,  it  was  customary  to  write  the  symbols 
as  in  this  last  reaction,  separating  the  acid  from  the 
basic  oxide  by  a  comma.  Here  are  a  few  other  exam- 
ples : 

CaO,SO3  FeO,SO3  ZnO,N2O6. ' 

Sulphate  of  Lime.  Sulphate  of  Iron.  Nitrate  of  Zinc. 

As  expounded  and  illustrated  by  Berzelius,  the 
dualistic  theory  had  the  charm  of  great  simplicity,  and 
was  greatly  strengthened  by  the  electro-chemical  facts 
which  he  brought  forward  in  its  support.  The  division 
of  the  elementary  substances  into  electro-positive  and 
electro-negative  elements  corresponded  very  closely  to 

1  To  avoid  confusion,  all  our  symbols  stand  for  the  new  atomic 
weights,  and  this  must  be  remembered  in  comparing  these  formulas  with 
those  in  the  old  books. 


THE  DUALISTIC  THEORY.  319 

the  distinction  between  metals  and  metalloids.  Bases 
were  compounds  of  electro-positive  elements  with  oxy- 
gen ;  and  acids,  on  the  other  hand,  the  oxides  of  electro- 
negative elements.  Again,  among  these  binary  com- 
pounds the  basic  oxides  were  electro-positive,  and  the 
acid  oxides  electro  -  negative.  Moreover,  the  wider 
apart  in  their  electrical  relations,  the  stronger  was  seen 
to  be  the  tendency  of  both  the  elements  and  of  their 
oxides  to  combine,  arid,  just  as  the  metals  united  to 
metalloids,  so  bases  united  with  acids.  Thus  was  formed 
the  class  of  ternary  compounds,  called,  as  above,  salts.1 
Among  these,  also,  could  be  distinguished  a  similar  op- 
position of  relations,  although  less  marked,  to  that  be- 
tween bases  and  acids,  and,  from  the  union  of  two  salts, 
resulted  the  class  of  quaternary  compounds,  or  double 
salts.  In  this  way  the  theory  advanced  from  element- 
ary substances  to  the  most  complex  compounds  through 
the  successive  gradations  of  binaries,  ternaries,  and  qua- 
ternaries ;  the  elements  or  compounds  only  combining 
with  substances  of  the  same  order,  two  and  two  togeth- 
er, like  two  magnetic  poles,  or  two  electrified  bodies. 

This  dualistic  theory  was  certainly  a  most  admira- 
ble system,  and  served  the  purposes  of  a  rapidly-grow- 

1  The  word  salt  was  used  in  chemistry  very  early  to  describe  any 
saline  substance  resembling  externally  common  salt;  but,  under  the 
dualistic  system,  the  term  came  to  be  applied  to  that  class  of  compounds 
which  were  supposed  to  be  formed  by  the  union  of  basic  and  acid  oxides, 
as  described  above.  Absurdly  enough,  however,  common  salt  was  thus 
ruled  out  of  the  very  class  of  compounds  of  which  it  had  previously  been 
regarded  as  the  type,  and  Berzelius,  in  his  electro-chemical  classification, 
made  a  distinct  family  of  those  substances  which  resemble  common  salt 
in  their  chemical  composition,  and  called  it  the  haloids.  But  this  name 
— bodies  resembling  salt—  only  rendered  the  anomaly  the  more  glaring, 
and  it  was  always  a  blemish  on  the  dualistic  system.  In  the  modern 
chemistry,  the  word  salt,  although  still  used  as  a  descriptive  name,  has 
no  technical  meaning. 


320  ELECTRO-CHEMICAL  THEORY. 

ing  science  for  more  than  half  a  century.  We  now 
feel  assured  that  the  old  theory  undervalued  essential 
circumstances,  and  misinterpreted  important  facts.  We 
maintain  that  hydrogen  is  an  essential,  not  an  accident- 
al constituent  of  all  acids  and  all  alkalies,  and  that, 
when  the  alkali  is  neutralized  by  the  acid,  the  reaction 
consists  in  the  replacement  of  this  hydrogen,  and  not 
in  the  direct  union  of  two  oxides.  Nevertheless,  given 
the  old  facts,  the  old  theory  was  logical  and  consistent, 
and  it  is  no  longer  tenable,  not  because  the  old  facts 
have  changed,  but  simply  because  a  whole  new  order 
of  facts  has  been  discovered  by  which  the  old  facts  must 
be  interpreted.  During  the  last  twenty-five  years  there 
has  been  discovered  a  great  mass  of  truths,  connected 
chiefly  with  the  compounds  of  carbon,  in  what  was  for- 
merly called  the  domain  of  organic  chemistry,  and  this 
is  to-day  the  most  prominent  and  attractive  portion  of 
our  science.  Moreover,  the  law  of  Avogadro  and  the 
doctrine  of  quantivalence  are  two  new  principles  which 
our  modern  science  has  added  to  the  old  chemistry,  and 
these  principles  have  supplanted  the  dualistic  theory. 
Let  us  not,  however,  undervalue  the  old  theory.  It 
was  an  important  stage  in  the  progress  of  science,  and 
a  noble  product  of  human  thought.  Theories  are 
means,  not  ends ;  but  they  are  the  appointed  means  by 
which  man  may  raise  himself  above  the  low  level  of 
merely  sensuous  knowledge  to  heights  where  his  intel- 
lectual eye  ranges  over  a  boundless  prospect  which 
it  is  the  special  privilege  of  the  student  to  behold. 
What  though  his  vision  be  not  always  clear,  and  his 
imagination  fill  the  twilight  with  deceptive  shapes 
which  vanish  as  the  light  of  knowledge  dawns ;  yet, 
to  have  enjoyed  the  intellectual  elevation,  is  reward 
enough  for  all  his  devotion  and  all  his  toil. 


LECTURE  XIV. 

ISOMERISM,    AND   THE    SYNTHESIS    OF    ORGANIC    COMPOUNDS. 

HAYING,  in  the  previous  lectures  of  this  course,  made 
you  familiar  with  the  conception  that  the  molecules  of 
every  substance  have  a  definite  atomic  structure,  which 
is  a  legitimate  object  of  scientific  investigation,  I  en- 
deavored in  my  last  lecture  to  illustrate,  by  numerous 
examples,  the  mode  now  generally  employed  in  chem- 
istry of  exhibiting  this  structure  by  means  of  what  are 
called  structural  formulae,  and,  during  the  whole  course 
of  these  lectures,  it  has  been  a  chief  object  to  develop 
the  fundamental  principles  on  which  these  formulae  are 
based,  in  order  that,  having  reached  this  stage,  you  might 
be  able  to  see  for  yourselves  that  they  were  legitimately 
deduced  from  the  facts  of  observation.  I  have  freely  ad- 
mitted that  they  were  the  expression  of  theoretical  con- 
ceptions which  we  could  not  for  a  moment  believe  were 
realized  in  Nature  in  the  concrete  forms,  which  our  dia- 
grams embody.  But  I  have  claimed  that  they  were  at 
present  our  only  mode  of  representing  to  the  mind  a 
large  and  important  class  of  facts,  and  were  to  be  val- 
ued as  the  first  glimpses  of  some  great,  general  truth, 
toward  which  they  direct  our  investigation.  Theories 
are  the  only  lights  with  which  we  can  penetrate  the 


322  ISOMERISM. 

obscurity  of  the  unknown,  and  they  are  to  be  valued 
just  so  far  as  they  illuminate  our  path.  This  ability  to 
lead  investigation  is  the  only  true  test  of  any  theory, 
and  it  will  be  my  object  in  this  lecture  to  show  that 
the  modern  chemical  theory  of  molecular  structure  has 
a  claim  to  be  regarded  as  one  of  the  most  valuable  aids 
to  discovery  which  science  has  ever  received. 

The  illustrations  of  molecular  structure  thus  far 
studied  have  been  mostly  taken  from  those  classes  of 
compounds  long  known  in  chemistry  under  the  names 
of  acids,  bases,  and  salts,  and  they  were  selected  be- 
cause it  was  with  such  substances  that  the  old  theory 
had  almost  exclusively  to  deal,  and  they  were  therefore 
the  best  adapted  to  illustrate  the  differences  between 
the  new  and  the  old  chemistry.  But,  as  I  have  already 
said,  the  strongest  evidence  in  favor  of  the  new  theory 
is  to  be  obtained  from  a  class  of  substances  about 
which  the  old  chemistry  knew  almost  absolutely  noth- 
ing, and  whose  number  has  been  enormously  increased 
during  the  past  twenty-five  years.  Indeed,  the  modern 
theory  is  so  completely  the  outgrowth  of  new  discov- 
eries that,  given  alone  the  old  facts,  the  question  be- 
tween the  old  and  the  new  theories  would  be  at  least 
of  doubtful  issue,  even  if  the  new  could  ever  have  been 
conceived.  The  class  of  substances  to  which  I  refer 
are  the  compounds  of  the  elementary  substance  car- 
bon. The  number  of  known  compounds  of  this  one 
element  is  far  greater  than  that  of  all  the  other 
elements  besides,  and  these  compounds  exhibit  a 
great  diversity  in  their  molecular  structure,  which  is 
often  highly  complex.  As  a  rule  they  consist  of  a 
very  few  chemical  elements  (besides  carbon,  only  hy- 
drogen, oxygen,  and  nitrogen),  but  the  number  of 
atoms  united  in  a  single  molecule  may  be  very  large. 


ORGANIC  COMPOUNDS.  323 

sometimes  even  exceeding  one  hundred.  Carbon  is 
peculiarly  the  clement  of  the  organic  world,  for,  leav- 
ing out  of  view  the  great  mass  of  water  which  liv- 
ing beings  always  contain,  organized  material  consists 
almost  exclusively  of  carbonaceous  compounds.  Hence 
these  substances,  with  the  exception  of  a  few  of  the 
simplest,  were  formerly  called  organic  compounds,  and 
in  works  on  chemistry  they  are  usually  studied  to- 
gether under  the  head  of  organic  chemistry.  It  was 
formerly  supposed  that  the  great  complexity  of  these 
substances  was  sustained  by  what  was  called  the  vital 
principle;  but,  although  the  cause  which  determines 
the  growth  of  organized  beings  is  still  a  periect  mys- 
tery, we  now  know  that  the  materials  of  which  they 
consist  are  subject  to  the  same  laws  as  mineral  mat- 
ter, and  the  complexity  may  be  traced  to  the  pe- 
culiar qualities  of  carbon.  In  like  manner  the  notion 
that  these  so-called  organic  substances  owed  their  ori- 
gin to  some  mysterious  energy,  which  overruled  the 
ordinary  laws  of  chemical  action,  for  a  long  time  pre- 
cluded from  the  mind  of  the  chemist  even  the  idea 
that  they  could  be  formed  in  the  laboratory  by  purely 
chemical  processes ;  so  that,  although  the  analysis  of 
these  compounds  was  easily  effected,  the  synthesis  was 
thought  impossible.  But  within  a  few  years  we  have 
succeeded  in  preparing  artificially  a  very  large  number 
of  what  were  formerly  supposed  to  be  exclusively 
organic  products;  and  not  only  this,  but  the  processes 
we  have  discovered  are  of  such  general  application  that 
we  now  feel  we  have  the  same  command  over  the  syn- 
thesis of  organic,  as  of  mineral  substances.  The  chem- 
ist has  never  succeeded  in  forming  a  single  organic  cell, 
and  the  whole  process  of  its  growth  and  development  is 
entirely  beyond  the  range  of  his  knowledge ;  but  he 


324  ISOMERISM. 

has  every  reason  to  expect  that,  in  the  no  distant  future, 
he  will  be  able  to  prepare,  in  his  laboratory,  both  the 
material  of  which  that  cell  is  fashioned,  and  the  various 
products  with  which  it  becomes  filled  during  life. 

The  number  of  elements  which  enter  into  the  com- 
position of  organic  compounds  being  so  restricted,  it 
is  evident  that  the  immense  variety  of  qualities  which 
they  present  cannot  be  referred  solely  to  the  influence 
of  the  simple  radicals  which  they  contain.1  Moreover, 
there  appears  among  these  organic  substances  a  most 
remarkable  phenomenon,  which,  although  not  unknown 
in  the  mineral  kingdom,  is  peculiarly  characteristic 
of  these  complex  compounds  of  carbon.  We  are  ac- 
quainted with  a  large  number  of  cases  of  two  or  more 
wholly  different  substances  having  exactly  the  same 
composition  and  the  same  vapor  density.  Here,  for 
example,  are  two  such  substances  : 

The  first,  butyric  acid,  is  an  oily  liquid  with  whose 
smell  we  are  only  too  familiar,  since,  when  formed  in 
rancid  butter,  it  imparts  to  this  article  of  our  food  its 
peculiarly  offensive  odor.  But,  though,  as  the  odor 
shows,  it  must  slowly  volatilize  at  the  ordinary  tem- 
perature, it  does  not  boil  lower  than  156°  C.,  and  does 
not  easily  inflame.  Further,  as  its  name  denotes,  it 
has  the  qualities  of  an  acid,  reddening  litmus-paper, 
and  causing  an  effervescence  with  alkaline  carbonates. 

Utterly  different  from  this  offensive  acid  is  the  sec- 
ond substance,  which  we  call  acetic  ether,  a  very  lim- 
pid liquid,  with  a  pleasant,  fruity  smell,  highly  volatile, 
boiling  at  74°  and  inflaming  with  the  greatest  ease. 
Notice,  also,  that  it  does  not  in  the  least  affect  the 
colors  of  these  sensitive  vegetable  dyes. 

Yet,  butyric  acid  and  acetic  ether  have  exactly  the 

1  Compare  pages  273  and  293. 


THE  QUESTION  STATED.  325 

same  composition,  and  the  same  vapor  density.  The 
results  both  of  actual  chemical  analysis  and  of  the 
determination  of  vapor  density  are  given  in  this  dia- 
gram, and  the  figures  obtained  in  the  two  cases  do 
not  differ  more  than  we  should  expect  the  results  of 
different  analyses  of  the  same  substances  to  differ ;  for 
it  must  be  remembered  that,  in  such  experimental 
work,  we  can  only  attain  a  certain  degree  of  accuracy, 
and  that  we  may  disregard  all  variations  which  are 
within  the  limit  of  probable  error : 

Analyses  of  Isomeric  Compounds. 

By  Grunzweig.  By  Liebig. 

Butyric  Acid —  Acetic  Ether — 

Carbon 54.51                  Carbon 54.47 

Hydrogen 9.26                  Hydrogen 9.67 

Oxygen 36.23                  Oxygen 35.86 


100.00  100.00 

By  Cahours.  By  Boullay  and  Dumas. 

Sp.  Gr 44.3  Sp.  Gr 44.1 

Molec.  weight 88.0  Molec.  weight 88.0 

If,  now,  from  these  experimental  results,  we  come 
to  calculate  the  symbols  of  the  two  substances,  accord- 
ing to  the  method  I  have  so  fully  described,  we  shall 
obtain  in  both  cases  precisely  the  same  formula,  C4H8O2, 
and  it  must,  therefore,  be  that  the  molecules  of  these 
two  substances  contain  the  same  number  of  atoms  of 
the  same  three  elements,  carbon,  hydrogen,  and  oxygen. 
Here,  then,  we  come  face  to  face  with  a  most  remarkable 
fact.  For,  to  affirm  no  more  than  can  be  absolutely 
demonstrated,  this  pleasant  odor  of  apples  and  this  dis- 
gusting smell  of  rancid  butter  come  from  substances 
consisting  of  the  same  elements  united  in  the  same 
proportions.  What,  then,  can  be  the  cause  of  the  dif- 
ference ?  We  cannot  allow  such  a  fundamental  fact  as 


326  ISOMERISM. 

this  to  pass  unchallenged.  It  is  evident  that  there  is 
an  all-important  condition  which  has  escaped  our  ele- 
mentary analysis.  The  circumstances  demand  investi- 
gation, and  it  would  be  a  disgrace  to  our  science  not  to 
attempt  to  answer  the  question.  Can  you  wonder, 
then,  that,  for  the  past  ten  years,  a  great  part  of  the 
intellectual  force  of  the  chemists  of  the  world  has  been 
applied  to  the  problem,  and  in  this  course  of  lectures  I 
have  been  endeavoring  to  present  to  you  the  result 
they  have  reached.  The  answer  they  have  obtained 
is,  that  the  difference  of  qualities  depends  on  molecu- 
lar structure,  and  that  the  same  atoms  arranged  in  a 
different  order  may  form  molecules  of  different  sub- 
stances having  wholly  different  qualities.  But  they 
have  gained  more  than  this  general  result. 

These  isomeric  compounds,  as  we  call  them,  when 
acted  on  by  chemical  agents,  break  up  in  very  different 
ways,  and,  by  studying  the  resulting  reactions,  we  are 
frequently  able  to  infer  that  certain  groups  of  atoms 
(or  compound  radicals)  are  present  in  the  compounds, 
because  we  know  that  they  exist  in  the  products  which 
these  compounds  respectively  yield  ;  our  knowledge  of 
the  structure  of  these  very  radicals  probably  depending 
on  yet  other  reactions,  by  which  they  again  may  be  re- 
solved into  still  simpler  groups. 

Thus,  for  example,  if  we  act  on  acetic  ether  with 
potassic  hydrate,  we  obtain  two  products,  potassic  ace- 
tate and  common  alcohol.  Now,  we  know  that  alcohol 
has  the  symbol  C2H5-0-H  and  contains  the  radical 
C2H5,  which  we  call  ethyl.  Farther,  we  know  that 
potassic  acetate  has  the  symbol  K-O-(C2H3O)  and  con- 
tains the  radical  C2H3O,  which  we  call  acetyl.  Hence 
we  infer  that  the  ether  contains  both  of  these  groups, 
and  that  its  symbol  must  be  C2H5-O-C2H3O.  The  reac- 


STRUCTURE  OF  ACETIC  ETHER. 


327 


tion  obtained  with  potassic  hydrate  is,  then,  seen  to 
consist  in  a  simple  metathesis  between  K  and  C2H5. 


C2H6-O-C2H3O 

Acetic  Ether. 

K-O-H 

Potassic  Hydrate. 


K-O~C2H3O 

Potassic  Acetate. 

C2H5-0-H 

Alcohol. 


Passing  next  to  the  radical  ethyl  C2H5,  we  can  show 
that  it  may  be  formed  in  a  compound  which  contains 
the  radical  CH3,  called  methyl,  by  substituting  for  one 
of  the  hydrogen-atoms  of  this  radical  another  group  of 
the  atoms  CH3,  thus : 


H 

H-6-X 

i 
H 

First  Methyl  Compound. 

H 

H-O-Y 

i 

H 
Second  Methyl  Compound. 


H    H 

H-C-C-X 

i      i 

H    H 

Ethyl  Compound. 


H-Y 

Hydrogen  Compound. 


In  this  assumed  reaction  the  terminal  hydrogen- 
atom  of  the  first  methyl  compound  changes  place  with 
the  methyl  radical  of  the  second,  thus  producing  the 
compounds  in  the  second  column.  Such  a  reaction  can 
actually  be  produced  with  a  variety  of  substances,  and 
these  symbols  may  be  supposed  to  stand  for  any  of  the 
substances  between  which  the  reaction  is  possible.  We 
use  X  and  Y,  instead  of  writing  the  symbols  of  definite 
compounds,  in  order  to  confine  the  attention  to  the 
change  which  takes  place  in  the  radical  alone. 

In  reactions  of  this  kind  we  form  the  radical  ethyl 
in  such  a  way  as  to  leave  no  doubt  whatever  in  regard 
to  its  structure,  and  in  a  precisely  similar  way  we  have 


328  ISOMERISM. 

worked  out  the  structure  of  acetyl.  We  represent  the 
structure  in  the  two  cases  thus  : 

H  H  OH 

ii  H      i 

H-O-C-  -C-C-H 

i     i  i 

H   H  H 

Ethyl.  Acetyl. 

Hence  we  conclude  that  the  structure  of  a  molecule 
of  acetic  ether  should  be  represented  as  follows : 

H  H         OH 

ii          ii      i 
H-C-C-0-C-C-H 

i      i  i 

H  H  H 

Acetic  Ether. 

Moreover,  since  we  are  led  to  the  same  result,  whether 
we  study  the  reactions  by  which  the  ether  may  be  pre- 
pared or  those  by  which  it  may  be  decomposed,  we  feel 
great  confidence  in  our  result. 

If,  now,  we  act  on  butyric  acid,  the  isomer  of  acetic 
ether,  with  potassic  hydrate,  the  same  reagent  as  before, 
we  obtain  wholly  different  products.  They  are  potas- 
sic butyrate  and  water;  and  here  the  knowledge  of 
acids,  bases,  and  salts,  which  we  obtained  at  the  last 
lecture,  comes  in  to  help  us  interpret  the  reaction.  It 
must  be  simply  as  follows : 

H-0-C4H70          )  (          K-0-C4H70 

Butyric  Acid.  1  Potassic  Butyrate. 

K-O-H  f  1  H-O-H 

Potassic  Hydrate.          )  \  Water. 

Evidently,  then,  butyric  acid,  instead  of  containing  the 
two  radicals  C2H5  and  C2H3O,  like  acetic  ether,  contains 
the  more  complex  radical  C4H7O,  and  the  simple  radi- 
cal H. 

But,  although  the  last  reaction  shows  that  butyric 
acid  contains  the  radical  C4H7O,  it  gives  us  no  infor- 


STRUCTURE  OF  BUTYRIC  ACID.  329 

mation  in  regard  to  the  grouping  of  the  atoms  in  the 
radical.  Of  course,  we  have  sought  to  discover  what 
the  structure  is,  and  the  result  of  the  investigation  is 
most  remarkable,  for  it  appears  that  there  are  two  dif- 
ferent radicals  having  the  same  composition  and  corre- 
sponding to  two  distinct  varieties  of  butyric  acid,  which 
differ  in  their  odor,  their  boiling-point,  and  other  quali- 
ties, and,  further,  various  reactions  show  that  the  atoms 
of  the  radicals  are  arranged  in  the  two  acids  as  the  fol- 
lowing formulae  indicate : 

H 

O    H  H  H  O  H-C-H 

H      i     t     i  H          i 

H-0-C-O-C-C-H  H-O-C C-H 

iii  i 

H  H  H  H-C-H 

Normal  Butyric  Acid.  I 

(Prepared  from  butter  or  by  fermentation.)  H 

Isobutyric  Acid. 
(A  product  of  synthesis.) 

There  are,  therefore,  at  least  three  substances  having 
the  composition  C4H8O2. 

Now,  by  studying  in  a  similar  way  the  whole  scheme 
of  carbon  compounds,  and  connecting  by  reactions  the 
more  complex  with  the  simpler,  it  has  been  found  pos- 
sible, in  a  very  large  number  of  instances,  to  deter- 
mine the  manner  in  which  the  atoms  are  grouped  in 
the  respective  molecules,  and  thus  to  show  what  the 
variations  of  structure  are  which  determine  the  differ- 
ence of  qualities  in  these  isomeric  bodies.  Moreover, 
having  discovered  how  the  atoms  are  grouped,  it  has 
been  found  possible,  in  many  cases,  to  reproduce  the  com- 
pounds ;  and,  more  than  this,  chemists  have  frequently 
been  led  to  the  discovery  of  wholly  new  bodies,  isomeric 
with  old  compounds,  by  studying  the  possible  variations 
of  the  structural  symbol.  This  last  fact  has  such  an  im- 
portant bearing  on  our  subject,  tending  greatly  to  sub- 
23 


330  ISOMERISM. 

stantiate  the  general  truth  of  our  theory  of  molecular 
structure,  that  a  few  illustrations  will  be  interesting. 
One  of  these  we  have  already  seen,  for  the  isomeric 
modification  of  butyric  acid,  we  have  just  been  dis- 
cussing, was  foreseen  by  theory  before  it  was  discov- 
ered, and  it  is,  therefore,  an  example  in  point,  but 
there  are  many  other  cases  of  the  kind  which  are 
equally  remarkable. 

Butyric  acid  is  the  fourth  body  in  that  series  of 
volatile  acids  before  mentioned  (page  308),  of  which 
formic  and  acetic  acids  are  the  first  and  second  mem- 
bers. It  was  then  said  that  the  molecules  of  these  acids 
increase  in  weight  by  successive  additions  of  CH2  as 
we  descend  in  the  series,  and  it  has  been  shown  since 

H  H 

i     i 

(page  327),  that  the  radical  ethyl,  -C-C-H,  may  be 

H  H 

H 

i 

derived  from  methyl,  -  C  -  H,  by  replacing  the  terminal 

H 
H  by  another  methyl  group.     It  is  obvious  that  this 

H  H  H   H   H 

it  iii 

process  repeated  on  -C-C-H  would  give  -C-C-C-H, 

H   H  H   H   H 

and  that  the  result  of  successive  replacements  of  the 
same  kind  would  be  a  series  of  hydrocarbon  radicals 
differing  from  each  other  by  CH2  like  the  volatile  acids 
mentioned  above.  Furthermore,  it  is  equally  obvious 
that,  theoretically  at  least,  the  same  process  might  be 
applied  to  any  compound  containing  a  hydrocarbon 
radical ;  and  you  will  not  be  surprised,  therefore,  to 
learn  that  there  are  many  series  of  carbon  compounds, 
between  whose  members  we  find  this  same  common 


HOMOLOGOUS  COMPOUNDS. 


331 


difference.  Bodies  so  related  are  said  to  be  the  homo- 
logues  of  each  other ;  and  of  these  homologous  series, 
so  called,  no  one  has  been  more  carefully  studied  than 
that  of  the  volatile  acids,  of  which  nineteen  members 
are  known. 

Now,  it  is  obvious  that,  as  the  hydrocarbon  radical 
in  the  series  of  volatile  acids  increases  in  complexity, 
the  possibilities  of  varying  the  atomic  grouping  in- 
crease also.  Next  to  butyric  acid,  C4H8O2,  comes  va- 
leric acid,  C5H10O2,  and,  while  we  had  only  two  butyric 
acids,  we  can  have  four  valeric  acids,  whose  molecular 
structure  is  indicated  by  the  following  symbols  : 


O   H   H   H  H 
ii     i      i      i     i 
H-O-C-C-O-C-O-H 

i      i      i      i 
H   H    H    H 

Normal  Valeric  Acid. 


H-O-C-C 


H 

O   H  H-6-H 

i  i 


0-H 

H  H-6-H 

i 

H 

Isovaleric  Acid. 


H 

O  H-O-H  H 
H-O-C 6 0-H 

H-6-H  H 

i 
H 

Third  Form. 


H 

O  H-6-H  H   H 
ii  i  ii 

H-O-0 0 0-0- H 

i  i      i 

H         H    H 

Fourth  Form. 


When  the  first  edition  of  this  book  was  published, 
only  three  of  these  possible  modifications  of  valeric 
acid,  pointed  out  by  theory,  had  been  described ;  but 
the  fourth  has  since  been  discovered,  and  all  are  now 
known.  Examples  similar  to  this  are  already  numer- 
ous, and  are  rapidly  multiplying,  but  I  have  only  time 
to  cite  one  other  instance. 


332  ISOMERISM. 

A  compound  called   cyanic  ether  has  long  been 
known,  and  its  symbol  was  always  assumed  to  be — 

(0»H6)-O-C=N, 

after  the  analogy  of  the  other  ethers ;  that  is,  it  was 
assumed  to  contain  the  compound  radicals,  ethyl,  C2H5, 
and  cyanogen,  ON,  united  through  an  atom  of  oxygen. 
But,  as  is  obvious,  we  may,  without  changing  the  radi- 
cal ethyl,  group  the  other  atoms  thus  : 

(C2H5)-N=C=0, 

and,  on  searching  for  this  substance,  an  isomer  of  the 
supposed  cyanic  ether  was  actually  obtained,  and  called 
cyanetholine.  Very  singularly,  however,  further  inves- 
tigation proved  that  the  new  compound  was  the  real 
cyanic  ether,  and  that  the  old  one  had  the  constitution 
represented  by  the  last  symbol.  Evidently,  then,  we 
are  not  infallible ;  but  the  very  mistake  has  been  in- 
structive ;  for,  in  detecting  and  correcting  the  error, 
we  have  the  more  clearly  shown  that  our  methods  are 
trustworthy. 

I  hope  I  have  been  able  to  give  some  general  no- 
tions of  the  manner  in  which  we  have  obtained  our 
knowledge  of  the  grouping  of  the  atoms  in  the  com- 
pounds of  carbon.  More  than  this  cannot  be  expected 
in  a  popular  lecture ;  for,  so  interwoven  is  the  web  of 
evidence  on  which  the  conclusions  are  based,  that,  to 
enter  into  full  details  in  regard  to  any  one  of  the  more 
complex  compounds,  would  be  wearisome,  and  the  work 
is  much  better  suited  for  the  study  than  the  lecture- 
room.  Indeed,  I  fear  that  I  have  already  imposed  too 
great  a  burden  on  your  patience  ;  but,  if  you  have  fol- 
lowed me  thus  far,  you  will  be  interested  in  some  of 
the  results  which  we  have  reached,  and  which  you  are 
now  prepared  to  understand.  I  must  necessarily  pre- 


CARBON  RADICALS.  333 

sent  these  results  as  they  have  been  formulated  by  our 
theory  of  atomic  bonds ;  for,  without  the  aid  of  these 
formulae,  we  cannot  either  think  or  talk  clearly  about 
the  subject. 

The  one  characteristic  of  carbon  on  which  the  great 
complexity  and  variety  of  its  compounds  depend  is,  the 
power  which  its  atoms  possess  of  combining  among 
themselves  to  an  almost  indefinite  extent.  As  a  rule, 
chemical  combination  takes  place  readily  only  between 
dissimilar  atoms.  It  is  true  that  we  have  met  with 
many  examples  of  the  union  of  similar  atoms,  as  in  the 
molecules  of  several  of  the  elementary  gases,  like — 

H-H  C1-C1  O  =  O  ST  =  ET 

Hydrogen  Gas.        Chlorine  Gas.  Oxygen  Gas.         Nitrogen  Gas. 

So,  also,  in  the  compounds — 

01    Cl  O 

Cl-Fe-Fe-Cl  and  Al^Al 

it  s         \ 

01     Cl  GO 

Ferric  Chloride.  Aluminic  Oxide. 

and  likewise  in 

O 

Cl-Hg-Hg-Cl  and  Cu-Cu 

Mercurous  Chloride.  Cuprous  Oxide — 

two  atoms  are  united  by  a  single  bond,  forming  a  bi- 
nary group,  which  is  the  radical  of  the  metallic  com- 
pound. But,  in  all  these  cases,  the  power  of  combina- 
tion is  very  limited,  admitting  the  grouping  together 
of  only  a  very  few  atoms  at  the  most,  and  generally  of 
only  two.  The  carbon-atoms,  however,  not  only  unite 
with  each  other  in  large  numbers,  but  form  groups  of 
great  stability,  which,  in  organic  compounds,  take  the 
place  of  the  elementary  radicals  of  the  mineral  king- 
dom. Let  us  begin,  then,  by  constructing  these  radi- 
cals : 


334  SYNTHESIS  OF  ORGANIC  COMPOUNDS. 


\  I 


O  II      <» 

I 
*,        xox 

O  II 


I 

^  XOX 

Q^O'H         o'«   X0-        ii     <*     O  II 

I  I 

/  '  o  XQ\ 

5>  Illl  O     <*»  I  —  O       **         O  —  I0Q  H        CD  I 

Q'V                                  v            //                         r~)                /          V             T~\  7--\ 

r-\  N  r^  "  s  ^  w  __  '         N     /  v«/  \ ^  >^ 


1 


\    /°C       \o/°\ 

II     xo^;o-      ii    «    o 


r.      -o-  x       o     x  0 

LQ-  ^  „     X0  ^  ,     XQ  oo    ox  ^  ,    s    o 


1 

-0- 

1 

1 

-0- 

1 

-o- 

I 

1        -0- 
^-    <o    | 

-o- 

1 
te-Q- 

1 

-o- 

s       1 

-o- 

1 

«_o- 

1 

-o- 

J*        1 

1        -0- 

i 

-o- 

1 

-o- 

1 

-o- 

1 

1 

-o- 

I 

1 

-o- 

1 

-Q- 

1 

1 

-o- 

CARBON  RADICALS.  335 

The  carbon -atoms  being  quadrivalent,  they  may 
unite  with  each  other  either  by  one,  two,  three,  or  four 
bonds,  and  the  larger  the  number  of  bonds  which 
are  thus  closed,  the  less  will  evidently  be  the  com- 
bining power  of  the  resulting  radical.  Hence  may 
arise  radicals  like  those  represented  in  the  diagram 
on  the  previous  page.  It  is  evident  that  this  table 
might  be  extended  indefinitely,  but  the  number  of 
terms  given  is  sufficient  to  illustrate  the  simple  rela- 
tion between  the  several  radicals  thus  formed.  Each 
group  of  carbon-atoms  can  have  a  maximum  quantiva- 
lence  of  2n  -f  2  (the  letter  n  denoting  the  number  of 
carbon-atoms  in  the  group),  and  from  this  maximum 
the  quantivalence  may  fall  off  by  two  bonds  at  a  time 
until  it  is  reduced  to  zero.  Thus  we  have  for  the  six- 
atom  group  a  maximum  of  14 ;  but  the  same  group 
may  also  have  a  quantivalence  of  12,  10,  8,  6,  4,  or  2. 

The  symbols,  however,  given  in  the  table  do  not  by 

No.  1. 

2.  8. 

I  I 

i.  -0-  -0- 

I      I      I      I      I  III  III 

.0-0-0-0-0-  -0-0-0-  -0-0-0- 

I      I      I      I      I  III  III 

-0-  -0- 

I  I 

No.  2. 


1. 

2. 

fill 

1      1     1      1 

-0-0=0-0- 

0=0-0-0- 

1                1 

1           1     1 

3. 

4. 

-6- 

\  / 
0 

5. 

I      i 

\  /  \  / 

I 

0  =  0 

0      0 

A  A/0 

1      1 

/  \  /  \ 

-0-0     i 

-0- 

0 

i        N0 

1 

/  \ 

1 

any  means   exhaust   the  possibilities  of   combination 
with  the  given  number  of  carbon-atoms;  for  further 


336  SYNTHESIS  OF   ORGANIC   COMPOUNDS. 

variations  may  be  obtained  by  changing  the  relative 
position  of  the  atoms  while  retaining  the  same  quan- 
tivalence.  Thus,  the  radical  (C5)XI1  may  be  constructed 
in  the  several  ways  shown  in  diagram  No.  1,  and,  al- 
though the  several  radicals  thus  obtained  contain  the 
same  number  of  atoms,  and  have  the  same  quanti va- 
lence, they  are  fundamentally  different.  The  differ- 
ence consists,  not  in  the  mere  grouping  of  the  letters 
on  the  page,  which  is  purely  arbitrary,  but  in  the 
fact  that,  while  in  i  no  carbon-atom  is  united  with 
more  than  two  others,  in  2,  one  of  the  atoms  is  united 
with  three  others,  and,  in  s,  with  four.  As  the  num- 
ber of  atoms  in  the  group  increases,  the  number  of 
possible  variations  must  necessarily  become  very  great- 
ly augmented.  Moreover,  when  some  of  the  atoms  are 
united  by  double  bonds,  a  variation  may  be  obtained 
by  shifting  the  position  of  this  double  bond  as  well 
as  by  varying  the  position  of  the  atoms  with  respect 
to  each  other.  This  is  illustrated  by  diagram  No.  2, 
which  shows  the  possible  forms  of  the  group  (C^™. 
It  is  unnecessary,  however,  to  multiply  illustrations ; 
for  it  is  evident  that  a  great  multitude  of  radicals  may 
be  obtained  with  even  a  very  limited  number  of  car- 
bon-atoms, and  to  attempt  to  exhaust  the  possibilities 
would  be  an  endless  task.  Some  of  my  audience,  how- 
ever, may  be  interested  to  study  the  subject  further, 
and  I  would,  therefore,  set  them  as  a  problem  to  find 
the  number  of  possible  combinations  which  can  be 
made  with  a  group  of  six  carbon-atoms,  having  a  quan- 
tivalence  of  twelve.  Such  investigations  are  not  with- 
out their  profit ;  for,  although  many  of  the  possibilities 
may  not  be  realized  in  Nature,  yet  the  practice  will 
give  a  clear  idea  of  what  is  meant  by  an  essentially  dif- 
ferent structure.  It  may  hereafter  appear  that  changes 


CARBON   RADICALS.  337 

of  position  corresponding  to  the  upper  and  lower,  or 
the  left  and  right  hand  sides  of  our  diagram,  constitute 
really  essential  variations  of  structure ;  but,  although 
there  are  some  facts  looking  in  this  direction,  we  do 
not  as  yet  admit  that  any  such  differences  are  of  im- 
portance, and  we  regard  any  two  groups  as  the  same 
when,  by  any  change  that  does  not  alter  the  relative 
order  of  the  atoms,  or  the  number  of  bonds  by  which 
they  are  united,  the  two  can  be  made  to  coincide  thus  : 

i  i  \  /   / 

-0-  -0-  0-0 

lit..  Ill  1  x/      ^       I 

-C-C-C-isthesameas-C-0-C-,and  0       0- the  same 

'-o-  -c-'  /NC=C 

I  I  /     \ 

\     /  \     /  \    \/ 

0-0  0=0  0-0 

as  0        0,  but  not  the  same  as    0       0  or  0       0 

s\      /  \  /\    /\            \/\ 

0-0  0=0                0-0 

/\  /\  /     \ 


The  radicals  thus  formed  may  be  regarded  as  the  skel- 
etons of  the  organic  compounds.  These  carbon -atoms, 
locked  together  like  so  many  vertebrae,  form  the  frame- 
work to  which  the  other  elementary  atoms  are  fastened, 
and  it  is  thus  that  the  complex  molecular  structures, 
of  which  organized  beings  consist,  are  rendered  possi- 
ble ;  moreover,  when  we  remember  that,  while  the  ele- 
mentary substance  carbon  is  a  fixed  solid,  the  three 
elementary  substances,  oxygen,  hydrogen,  and  nitro- 
gen, with  which  it  is  usually  associated,  are  permanent 
gases  under  all  variations  of  terrestrial  climate,  this 
analogy  of  the  carbon-nucleus  to  the  skeleton  of  the 
vertebrate  animal  becomes  still  more  striking. 

Having  thus  shown  how  the  skeletons  may  be 
formed,  let  us  next  see  how  these  dry  bones  may  be 
clothed.  In  order  to  illustrate  this  point,  I  will  sim- 


338  SYNTHESIS  OF  ORGANIC  COMPOUNDS. 

ply  take  two  of  the  numberless  carbon-radicals,  which 
are  theoretically  possible,  and  show  how  from  them  a 
set  of  familiar  organic  products  can  be  derived.  Let 
the  two  be  the  radicals  represented  in  this  diagram  : 

i 

0 


I      I      I  _O        n 

-C-C-C-  \    ? 

i      I      .  0 

i 

To  such  carbon-skeletons  a  large  number  of  different 
elementary  atoms  and  compound  radicals  can  be  attached 
by  various  chemical  processes  ;  but  the  number  of  those 
usually  met  with  in  organic  compounds  is  very  limited, 
and  only  the  following  will  be  considered  in  this  con- 
nection, namely  : 

H-,        -0-,        H-0-,        g)N-,         g)N- 

Hydrogen.       Oxygen.         Hydroxyl.  Amidogen.  Nitryl. 

Indeed,  by  doubling  this  number,  we  could  obtain  the 
materials  for  constructing  nearly  the  whole  scheme  of 
modern  organic  chemistry. 

Beginning,  then,  with  the  nucleus  -  C  -  0  -  0  -,  let  us, 

in  the  first  place,  satisfy  all  the  open  bonds  with  hydro- 
gen-atoms. The  result  is  — 

H    H    H 

i      i      i 

H-C-C-C-H 

i       i      i 
H    H    H 

Propylic  Hydride. 

a  combustible  gas,  which  is  found  mixed  with  numer- 
ous other  compounds  of  the  same  class  in  our  petrole- 
um-wells. Propyl  hydride  is  the  third  in  a  series  of 
homologous  compounds,  of  which  no  less  than  nine 
have  been  identified  in  our  Pennsylvania  petroleums. 


PETROLEUMS.  339 

Methylic  hydride 0  H4  Gas. 

Ethylic  hydride C2H6  " 

Propylic  hydride C8H8  " 

Butylic  hydride C4Hi0  84° 

Amylic  hydride C6H12  98° 

Hexylic  hydride C6Hi4  160° 

Heptylic  hydride C7Hi6  208° 

Octylic  hydride. C8Hi8  255° 

tfonylic  hydride 09H2o  304° 

The  diagram,  above,  gives  their  names  and  boiling- 
points.  Our  common  kerosene  is  chiefly  a  mixture 
of  hexylic  and  heptylic  hydride,  and  the  light  naphthas 
a  mixture  of  amylic  and  hexylic  hydrides.  Notice 
here,  again,  the  common  difference,  CH2,  between  the 
symbols  of  any  two  consecutive  members  of  this  series 
of  hydrocarbons. 

If,  next,  we  substitute  an  atom  of  oxygen  for  two 
of  the  hydrogen-atoms  which,  in  propylic  hydride,  are 
united  to  either  of  the  terminal  atoms  of  the  carbon- 
nucleus,  we  obtain  a  compound  called  propylic  alde- 
hyde. This  is  a  member  of  another  series  of  homo- 
logues,  parallel  to  the  last,  and  of  which  nearly  as  many 
members  are  known.  The  aldehydes,  as  these  bodies 
are  all  called,  have  very  striking  and  characteristic  qual- 
ities ;  and  these  qualities  may  be,  to  a  great  extent, 
traced  to  their  peculiar  molecular  structure.  If  we 
only  make  so  small  a  change  as  to  transfer  the  oxygen- 
atom  from  the  terminal  to  one  of  the  central  atoms  of 
the  carbon-nucleus,  we  obtain  a  class  of  compounds 
which,  though  isomeric  with  the  aldehydes,  have  wholly 
different  qualities,  and  are  called  ketones.  The  ketone 
isomeric  with  propylic  aldehyde  is  called  acetone  : 

H   H    O  H    O    H 

i      i      ii  i      H      i 

H-C-C-C-H  H-C-C-C-H 

ii  ii 

H    H  H          H 

Propylic  Aldehyde.  Acetone. 


340  SYNTHESIS  OF  ORGANIC  COMPOUNDS. 

Going  back  again  to  the  hydrocarbon,  CSH8,  and 
replacing  either  of  the  terminal  hydrogen-atoms  by 
the  radical  hydroxyl  (-O-H),  we  obtain  one  of  a  very 
important  class  of  compounds,  called  alcohols. 

H    H    H  H    H    H 

H-C-6-C-H  H-C-C-C-0-H 

iii  iii 

H    H    H  H    H    H 

Propylic  Hydride  gives  Propylic  Alcohol. 

Propylic  alcohol  is  the  third  member  of  still  another 
series  of  homologous  compounds,  of  which  our  common 
alcohol  is  the  second  member. 

Normal  Alcohols. 

Methylic  alcohol  (wood-spirit) C  Hg  -O-H 

Ethylic  alcohol  (common  alcohol) C2  H5  -O-H 

Propylic  alcohol C3  H7  -O-H 

Butylic  alcohol C4  H9  -O-H 

Amylic  alcohol  (fusel-oil) C5  Hn-O-H 

Hexylic  alcohol C6  His-O-H 

Heptylic  alcohol C7  Hi6-O-H 

Octylic  alcohol C8  Hn-O-H 

The  structure  of  the  alcohol  may  obviously  be 
varied,  like  that  of  the  aldehyde,  by  transferring  the 
hydroxyl  from  the  terminal  to  one  of  the  central  atoms 
of  the  carbon-nucleus  ;  but  we  thus,  as  before,  obtain  a 
wholly  new  set  of  substances,  which,  although  resem- 
bling the  normal  alcohols  in  many  respects,  differ  from 
them  in  important  particulars.  There  is,  for  example, 
an  isopropylic  alcohol,  which  is  isomeric  with  the  nor- 
mal propylic  alcohol,  and,  like  it,  resembles  externally 
common  alcohol.  But  the  pseudo-alcohol,  as  we  call 
it,  boils  at  85°  Cent.,  while  the  normal  alcohol  boils  at 
97°,  and,  when  acted  on  by  chemical  agents,  yields 
wholly  different  products : 


ALCOHOLS.  341 

H    H    II  H    H    H 

H-C-C-C-O-H  H-C-C-C-H 

iii  ill 

H    H    H  H    O    H 

Propylic  Alcohol.  I 

Isopropylic  Alcohol. 

Continuing,  now,  this  process  of  clothing  the  car- 
bon-skeleton, let  us,  in  the  next  place,  substitute  for 
two  of  the  hydrogen-atoms  of  the  normal  alcohol  an 
atom  of  oxygen,  selecting  for  replacement  the  two  hy- 
drogen-atoms which  are  connected  with  that  terminal 
carbon-atom  to  which  the  hydroxyl  is  united  : 
H  H  H  H  H  O 

H-C-C-C-0- H  H-C~C-C-0-H 

iii  ii 

H   H   H  H   H 

Propylic  Alcohol  gives  Propionic  Acid. 

Now,  propionic  acid  is  the  third  member  of  that  ho- 
mologous series  of  volatile  acids  of  which  a  partial  list 
has  already  been  given  (page  308),  and  of  two  of  whose 
members  the  possible  variations  of  structure  have  al- 
ready been  discussed  (pages  328  and  331). 

Again,  we  may  substitute  in  propionic  acid  a  second 
oxygen  atom  for  two  of  the  remaining  atoms  of  hydro- 
gen, and  we  thus  obtain  a  liquid  body  called  pyruvic 
acid,  a  perfectly  definite  substance,  although  one  with 
which  I  can  give  you  no  familiar  associations  : 

H  H    O  H    O   O 

i      i     ii  i      ii     ii 

H-C-C-C-O-H  H-C-C-C-O-H 

i      i  i 

H    H  H 

Propionic  Acid.  Pyruvic  Acid. 

The  acids  and  alcohols  we  have  thus  far  formed 
around  our  three-atom  carbon-nucleus  have  been  all 
monatomic.  The  atomicity  of  a  compound,  you  re- 
member, is  determined  by  the  number  of  atoms  of  hy- 
drogen which  are  easily  replaced  by  metathesis,  and 


342  SYNTHESIS  OF  ORGANIC  COMPOUNDS. 

only  those  atoms  of  hydrogen  can  be  so  replaced  which 
are  united  to  the  carbon-nucleus  through  an  atom  of 
oxygen.  Hence,  with  one  hydroxyl  group  we  can  only 
produce  monatomic  compounds.  Use  two  hydroxyl 
groups,  and  we  can  form  around  the  same  skeleton  a 
number  of  diatomic  compounds.  The  following  are  a 
few  examples.  After  what  has  been  said,  the  symbols 
require  no  detailed  description  ;  but  it  must  be  remem- 
bered that  the  grouping  is  no  play  of  fancy,  and  that 
a  good  reason  can  be  given  for  the  position  of  every 
letter : 

H  H  H  H   H  H 

H-O-C-C-C-O-H  H-O-C-C-C-H 

lit  iii 

H   H   H  H   O    H 

Normal  Propyl  Glycol.  I 

u 

Propy]  Glycol. 

O    H   H  O   H   H 

H-O-C-C-C-O-H  H-O-C-C-C-H 

ii  ii 

H    H  OH 

Normal  Lactic  Acid.  I 

Common  Lactic  Acid. 

Attach  to  the  nucleus  three  hydroxyl  groups,  and  there 
result  triatomic  compounds,  among  which  is  a  very  fa- 
miliar substance : 

H   H   H 

H-O-C-C-C-O-H 

i      i      i 

H    O    H 

i 

H 
Glycerine. 

O   H   H  OHO 

H-O-C-C-C-O-H  H-O-C-C-C-O-H 

i  i  i 

OH  O 

i  i 

H  H 

Glyceric  Acid.  Tartronic  Acid. 


AN  OLD  ACQUAINTANCE.  343 

Lastly,  replace  the  three  terminal  hydrogen-atoms  of 
glycerine  by  nitryl  (NO2),  and  we  meet  again  an  old 

acquaintance : 

O         H  H   H         O 
ii  iii  ii 

ET-O-C-C-C-O-tf 

ii  iii  it 

O          H   O    H          O 

O  =  N=O 

Nitro-glycerine. 

I  think  that  this  last  symbol  will  not  now  appear  to 
yon  so  strange  as  when  I  first  called  your  attention  to 
it  a  few  lectures  back.  It  is  true  that  I  have  not  act- 
ually proved  that  this  grouping  of  the  letters  repre- 
sents the  structure  of  the  nitro-glycerine  molecule,  but 
I  have  led  you  to  a  point  where  you  are  prepared  to 
accept  it  as  a  definite  result  of  investigation,  and  can 
feel  assured  that  the  proofs  await  your  examination  in 
the  due  course  of  your  study.  You  can  now  understand 
more  clearly  than  before  how  it  is  that,  by  the  struct- 
ure of  the  molecule,  the  oxygen-atoms  are  kept  apart 
from  the  atoms  of  carbon  and  hydrogen  for  which  the 
fire-element  has  such  a  strong  affinity,  and  how  these 
atoms  rush  into  more  stable  combinations  when  the 
delicate  balance  of  forces,  on  which  the  structure  de- 
pends, is  disturbed. 

You  have  now  seen  what  a  number  of  distinct  com- 
pounds can  be  obtained  by  attaching  to  one  of  the  very 
simplest  of  the  carbon-nuclei  atoms  of  hydrogen  and 
oxygen  alone.  Almost  every  commutation  we  could 
make  with  these  few  atoms  is  actually  realized  in  a  defi- 
nite substance.  Of  course,  with  the  names  of  many  of 
these  bodies  you  have  no  association.  You  must  accept 
the  assurance  that  they  stand  for  definite  substances, 
and  that  our  symbols  represent  the  results  of  care- 
ful investigation,  and,  knowing  this,  you  can  gain  some 


344  SYNTHESIS  OF  ORGANIC  COMPOUNDS. 

conception  of  the  knowledge  we  have  acquired  of  the 
structure  of  this  class  of  compounds ;  and,  when  you 
add  to  this  that,  in  many  of  these  cases,  the  theory  has 
gone  before  discovery,  and,  by  suggesting  possible  com- 
mutations of  the  atoms,  has  prefigured  compounds  which 
were  subsequently  obtained,  you  must  admit  that,  rude 
and  unreal  as  our  representations  of  molecular  struct- 
ure may  be,  they  have  a  positive  value,  both  as  means 
of  classifying  facts  and  as  aids  to  new  discoveries. 

Lastly,  let  us  turn  our  attention  to  the  second  of  the 
two  carbon-skeletons,  whose  dry  bones  we  proposed  to 
clothe  with  the  features  of  definite  compounds.     The 
group  of  bodies  whose  molecules  contain,  as 
\       /      we  assume,  this  nucleus  (Fig.  32),  has  been 
//       \      very  fully  investigated  by  Professor  Kekule, 
/of  Bonn,  and  to  him  we  owe  the  theory  of 
C  =  C       their   structure  which   our   diagram  repre- 
FIG.  32.       sents.     It  may  appear  superfluous  for  me  to 
repeat  that,  in  such  diagrams,  the  only  es- 
sential points  are  the  relative  order  of  the  atoms  and 
the  number  of  the  bonds  ;  but  the  hexagonal  shape  in 
which  we  find  it  convenient  to  represent  on  our  page 
the  structure  of  this  nucleus  suggests  the  idea  of  defi- 
nite form  so  forcibly,  that  additional  caution  may  be 
needed  to  avoid  misconstruction. 

The  bodies  with  which  we  are  now  to  deal  are,  for 
the  most  part,  products  either  already  existing  in  coal- 
tar,  or  which  may  be  obtained  from  it  by  various  chem- 
ical processes.  Among  them  are  those  gorgeous  ani- 
line dyes  which,  within  a  comparatively  few  years,  have 
added  so  much  to  the  elegances  of  common  life.  From 
a  very  large  number  of  compounds,  I  can  only  select  a 
few  examples.  Still,  I  shall  not  restrict  the  selection 
to  compounds  whose  molecules  contain  only  six  carbon- 


ANILINE   COLORS.  345 

atoms,  but  I  shall  endeavor  to  show  that  molecules  of 
extreme  complexity  can  be  built  up  either  by  the  addi- 
tion of  hydrocarbon  radicals  to  the  nucleus  represented 
in  Fig.  32,  or  by  the  coaleascing  of  two  or  more  of 
these  nuclei  into  one.  As  I  have  not  time  to  enter  into 
details,  the  symbols  must,  to  a  great  extent,  be  allowed 
to  speak  for  themselves. 

Coal-tar  is  a  mixture  of  a  very  large  number  of  sub- 
stances whose  boiling-points  vary  from  80°  Cent,  upward. 
When  the  tar  is  distilled,  and  the  distillate  rectified, 
the  more  volatile  product  obtained  is  chiefly  a  mixture 
of  two  hydrocarbons— benzol  and  toluol.  This  mix- 
ture, the  commercial  benzol,  is  used  in  large  quantities 
for  the  preparation  of  the  aniline  dyes  : 

H    H  H   H 

ii  ii 

C-C  H     C-0 

H-C       VH  H-C-C        C-H 
\       /  I      \       / 

0=0  H     0=0 

ii  i      ; 

H    H  H    H 

Benzol.  Toluol. 

When  benzol  and  toluol  are  treated  with  strong  ni- 
tric acid  the  products  are  : 

H   H  H    H 

ii  ii 

C-0  O                                    H      C-C      O 

s.      \  i                                    i     s      \     \\ 

H-C        C-1ST  H-C-C        0-N 

\       /  ii                                     i      \       /      ii 

C  =  0  O                                   H     0  =  0      O 

ii  ii 

H   H  H    H 

NitrobenzoL  or  Nitrotoluol. 

When  nitrobenzol  and  nitrotoluol  are  acted  on  by 
nascent  hydrogen  (in  the  arts  a  mixture  of  iron-filings 

and  acetic  acid  is  used),  we  obtain  : 
24 


346  SYNTHESIS  OF  ORGANIC  COMPOUNDS. 

H    H  H    H 

ii  ii 

C-C  H                                     H      C-0      H 

/      v  i                                    i      ^      \     i 

H-C        C-IST  H-C-C        0-N 

\       /  i                                      i      \       /      i 

C  =  C  H                                     H      C  =  C      H 

ii  ii 

H    H  H    H 

Aniline,  or  Toluidine. 

When  the  mixture  of  aniline  and  toluidine,  obtained 
in  the  arts  from  commercial  benzol,  is  treated  with 
various  oxidizing  agents,  we  obtain  salts  of 


H 

H                        H 

H 

i 

i                           i 

i 

N 

0         H  H         0 

N 

\    x  \    x        \     /  ^ 

/    1 

H 

0        OHO 

0    H 

ii          i       i       ii 

i 

C        000 

C    H 

X     \    X     \    1     X     \    / 

\  i 

H 

COO 

0-H 

1                           1 

i 

H                       H 

H 

H       0       H 

"(/  ^G 

ii          i 

0        0 

x  \  x  \ 

H       C       H 

i 

H-N-H 

Eosaniline. 

Eosaniline  is  a  base  like  ammonia.  As  I  have  before 
stated,  when  the  molecule  NH3  unites  with  acids  to 
form  salts,  the  quantivalence  of  the  nitrogen-atom  ap- 
pears to  be  increased  by  two  bonds  which  bind  the 
atoms  of  the  acid  molecules  (see  page  268).  So,  when 
rosaniline  combines  with  acids,  the  atoms  of  the  acid 
molecule  join  to  one  or  the  other  of  the  nitrogen-atoms 
in  the  complex  molecule  of  this  base.  Moreover,  as 
there  are  three  of  these  nitrogen-atoms  in  the  molecule 
of  rosaniline.  it  can  bind  either  one,  two,  or  three  mole- 


ANILINE   COLORS.  347 

cules  of  acid ;  for  example,  it  can  unite  either  with  HC1, 
with  2HC1,  or  with  3HC1.  Thus,  there  may  be  formed 
three  classes  of  salts,  and  those  which  contain  the  small- 
est amount  of  acid  are  used  in  the  arts  as  coloring 
agents.  These  salts,  when  crystallized,  have  a  very 
brilliant  beetle-like  lustre,  and  yield  beautiful  rose-red 
solutions.  They  possess,  moreover,  a  most  wonderful 
coloring  power. 

Taking  only  a  few  crystals  (one  grain  in  weight)  of 
the  hydrochlorate  of  rosaniline,  called  fuchsine  in  com- 
merce, and,  first  rubbing  them  up  in  a  mortar  with 
some  alcohol,  I  will  pour  the  concentrated  solution,  into 
a  large  glass  jar,  holding  two  gallons  of  water,  and  you 
see  that  this  very  small  quantity  of  dye  shows  a  brilliant 
red  color  even  when  diffused  through  the  great  body 
of  liquid.  By  combining  the  base  w^ith  different  acids 
we  obtain  only  slight  differences  of  tints,  but  very 
marked  alterations  of  color  can  be  produced  in  another 
way. 

By  recurring  to  the  symbol  of  rosaniline,  it  will  be 
seen  that  there  are  six  hydrogen-atoms  directly  united 
to  the  three  atoms  of  nitrogen  which  the  radical  con- 
tains. Now,  it  is  possible  to  replace  either  one,  two, 
or  three  of  these  hydrogen-atoms  by  various  hydro- 
carbon radicals ;  such  as — 

-CH3  -C2H5  -C6H5; 

Methyl,  Ethyl,        or        Phenyl. 

and  we  thus  obtain  other  bases  whose  salts  are  violet 
or  blue,  the  blue  tint  increasing  with  the  degree  of 
replacement.  I  have  in  these  five  jars  solutions  of 
some  of  these  salts,  the  aniline  violets  of  commerce, 
and  they  will  illustrate  to  you  the  gradations  of  color 
we  can  obtain  by  the  replacements  I  have  described. 
Among  the  less  volatile  products  of  the  distillation 


348  SYNTHESIS   OF   ORGANIC   COMPOUNDS. 

of  coal-tar  is  the  compound  called  phenol  or  carbolic 
acid,  which  is  so  much  used  as  an  antiseptic  agent. 
Here  is  its  symbol,  and  also  the  symbol  of  another  com- 
pound, which  has  an  atom  of  nitrogen  in  its  carbon 
skeleton,  but  which  otherwise  has  a  structure  similar  to 
that  of  phenol,  and  is  derived  from  a  similar  source : 

H  H 

H         6         H  H         C         H 

\    ^    \    /  \    /    \    x 

CO  CO 

CO  6        C 

/    \    /    \  /    \    x    \ 

H         0         H  H         N         H 

i 
O 

H 

Phenol.  Pyridine. 

Associated  with  pyridine,  among  the  products  of 
the  destructive  distillation  not  only  of  coal-tar  but  also 
of  the  natural  alkaloids  and  other  complex  compounds 
of  nitrogen,  is  another  remarkable  compound  whose 
framework  may  be  regarded  as  formed  by  a  coalescing 
of  the  phenol  and  pyridine  rings  : 
H  H 


H 

C        C 

H 

\    ^ 

?    \    /    * 

*    / 

0 

C 

C 

i 

II 

i 

C 

C 

C 

/    s 

*    /    \    < 

^    \ 

H 

C         N 

H 

i 

H 

Quinoline. 

The  salts  of  this  artificial  base  quinoline  have  in 
part  at  least  the  valuable  medicinal  qualities  of  those  of 
quinine,  and  it  is  hoped  may  replace  it  in  pharmacy ; 
moreover,  since  both  quinoline  and  pyridine  are  the 
chief  products  of  the  decomposition  of  quinine,  we  have 


NAPHTHALINE.  349 

reason  to  expect  that  before  long  we  shall  be  able  to 
reverse  the  destructive  change,  and  prepare  artificially 
the  natural  alkaloid  itself,  which  of  all  the  medicinal 
agents  which  Nature  produces  is  probably  the  most 
valuable  to  man.  Further,  while  the  salts  of  quinolirie 
have  the  medicinal  qualities  referred  to,  those  of  pyri- 
dine  are  poisonous,  and  the  salts  of  quinine  appear  to 
owe  their  medicinal  power  to  their  relations  to  quino- 
line,  while  the  unpleasant  accompanying  effects  seem 
to  be  related  to  pyridine.  Hence,  it  is  possible  that  art 
may  here  excel  Nature,  and  give  us  a  febrifuge  with 
all  the  power  of  quinine,  without  its  poisonous  quali- 
ties. 

But  passing  from  predictions  to  actual  achievements, 
let  me  next  call  your  attention  to  the  graphic  symbol 
of  naphthaline,  one  of  the  least  volatile  products  obtained 
in  the  distillation  of  coal4ar>  and  whose  molecule  ap- 
pears to  be  formed  by  the  coalescing  of  two  molecules 
of  benzol : 

H       H 

H        0       0        H 
\  v   \   /   \    / 

000 

I  II  I 

000 

/     \    /    \    //    \ 

H        0        0         H 

i          t 

H       H 

Naphthaline. 

This  body  yields  a  very  large  number  of  derivatives 
having  the  same  general  structure,  some  of  which  have 
such  a  deep  color  that  they  can  be  used  as  dyes. 

Associated  with  naphthaline  in  coal-tar  is  a  still  less 
volatile  hydrocarbon,  called  anthracene,  which  may  be 
regarded  as  formed  by  the  coalescing  of  three  molecules 
of  benzol : 


350 


SYNTHESIS   OF  ORGANIC   COMPOUNDS. 


H        H       H 

i                    i 

H 

000 

H 

\               , 

'    \    /  \    /    s 

*    / 

0 

0 

0 

0 

i 

II 

II 

1 

0 

0 

0 

0 

/     * 

i       /       \'/        \       '4 

*    \ 

H 

000 

H 

H       H       H 

Anthracene. 

From  anthracene  has  been  derived  a  remarkable 
compound  called  anthraquinone,  whose  molecule,  as 
you  will  notice,  contains  two  atoms  of  oxygen  in  place 
of  the  two  atoms  of  hydrogen  which  in  the  molecule 
of  anthracene  are  united  to  the  two  carbon-atoms  of 
the  middle  group. 


H        0        H 

H        0        0        0 

H 

Xc^  ^c'  ^c'  %o 

i       ii       ii       i 

0000 

^L      ^  V       0     ^C^ 

i          ii         i 
H        0        H 

XH 

Anthraquinone. 

Lastly,  from  anthraquinone,  the 
have  been  obtained  : 

following  products 

H 

HOG 

i         ii         i 
H        0        0        0 

H 
/ 

G 

xo'  ^c/  ^c'  V 

1          II         II         1 
0000 

-R7    %C^    V    \/ 
i          ii          i 
H        G        H 

H 

Alizarine. 

PURPURINE.  351 

H 

H        O        6  H 

i          ii          i  / 

H         C        0        0         O 
\    /    \    /  \    /    \    / 

0  0        C        C 

1  II         II         II 

coco 

/     \     /     \  /     \    V     \ 

HOOCH 

i          ii          i 
H        O        O 
i 
H 

Purpurine. 

This  brings  us  to  one  of  the  latest  and  most  note- 
worthy results  of  our  science.  Alizarine  and  purpurine, 
but  chiefly  alizarine,  are  the  coloring  principles  of  the 
madder-root,  which  has  long  been  the  chief  dyestuff 
used  in  printing  calicoes.  But,  although  the  subject 
had  been  most  carefully  investigated,  there  was  for 
many  years  a  question  in  regard  to  the  exact  composi- 
tion of  these  substances. 

Shortly  before  the  first  edition  of  this  book  was 
published^  Graebe,  a  German  chemist,  while  investigat- 
ing a  class  of  compounds  called  the  quinones,  determined 
incidentally  the  molecular  structure  of  a  body  closely 
resembling  alizarine,  which  had  been  discovered  several 
years  before.  This  body  was  derived  from  naphthaline, 
and,  like  many  similar  derivatives,  was  reduced  back  to 
naphthaline  when  heated  with  zinc-dust.  This  circum- 
stance led  the  chemist  to  heat  also  madder-alizarine 
with  zinc-dust^  when,  to  his  surprise,  he  obtained  an- 
thracene* Of  coursej  the  inference  was  at  once  drawn 
that  alizarine  must  have  the  same  relation  to  anthracene 
that  the  allied  coloring-matter  bore  to  naphthaline,  and, 
more  than  this,  it  was  also  inferred  that  the  same  chemi- 
cal processes  which  produced  the  coloring-matter  from 
naphthaline,  when  applied  to  anthracene,  would  yield 


352  SYNTHESIS  OF  ORGANIC   COMPOUNDS. 

alizarine.  The  result  fully  answered  these  expecta- 
tions, and  now  alizarine  is  manufactured  on  a  large 
scale  from  the  anthracene  obtained  from  coal-tar,  and, 
singularly  enough,  the  artificial  alizarine,  like  the  nat- 
ural, is  mixed  with  more  or  less  purpurine,  which  some- 
what modifies  its  color. 

Here  are  two  pieces  of  cloth,  one  printed  with 
madder  and  one  with  artificial  alizarine,  and  it  would 
require  a  practised  eye  to  distinguish  between  them. 
It  is  true,  nevertheless,  that  the  artificial  alizarine,  as 
now  manufactured,  is  not  identical  with  the  madder- 
dye.  The  same  substance,  alizarine,  is  the  chief  con- 
stituent in  both  cases,  but  there  is  a  difference  in  the 
by-products  which  become  mixed  with  the  alizarine  in 
the  process  of  manufacture.  "We  have  already  said  that 
alizarine  is  accompanied  by  purpurine,  both  in  the 
madder-root  and  in  the  artificial  product ;  but  the  struc- 
tural formula,  before  given,  represents  the  constitution 
of  madder-purpurine  only.  This  substance  differs  from 
alizarine  in  containing  a  third  hydroxyl  (-O-H)  group, 
and  it  will  be  noticed  that  the  three  hydroxyl  groups 
are  represented  in  our  symbol  as  on  the  same  end-ring. 
Mixed  with  the  artificial  alizarine,  there  are  several 
varieties  of  purpurine,  and  these  differ  from  the  madder- 
purpurine,  as  well  as  from  each  other,  in  that  the  hy- 
droxyl groups  are  differently  disposed,  not  always  on  the 
same  ring.  In  consequence  of  certain  varieties  of  color, 
caused  by  the  presence  of  these  isomeric  conditions  of 
purpurine,  the  artificial  alizarine  is  in  some  respects  a 
superior  dye  to  madder  itself.  It  is  also  true  that  all  the 
theoretical  considerations  which  led  Graebe  to  the  dis- 
covery have  not  proved  to  be  correct,  and  that  the  process 
of  manufacture  now  employed  is  quite  different  in  its  de- 
tails from  that  which  he  invented.  Nevertheless,  the  gen- 


GREAT  ACHIEVEMENTS.  353 

eral  features  of  the  process  are  the  same ;  and  there  is 
no  question  that  an  artificial  product  which  consists 
chiefly  of  alizarine,  and  has  to  a  great  extent  replaced 
the  madder-root  in  calico-printing,  is  now  manufactured 
by  a  method  which  was  first  pointed  out  by  theoretical 
science. 

This  certainly  is  a  most  remarkable  achievement. 
A  highly  complex  organic  product  has  been  actually 
constructed  by  following  out  the  indications  of  its  mo- 
lecular structure,  which  the  study  of  its  reactions,  and 
those  of  allied  compounds,  had  furnished.  It  is  a  re- 
sult that  all  can  appreciate,  and  which  the  world  will 
accept  as  the  most  trustworthy  credential  that  the  molec- 
ular theory  of  chemistry  could  offer.  The  circumstance 
that  this  substance  is  the  important  madder-dye,  and 
that  the  new  process  has  a  great  commercial  value,  of 
course,  really  adds  nothing  to  the  force  of  the  evidence 
in  favor  of  the  theory.  To  the  scientific  mind  the  evi- 
dence of  any  one  of  hundreds  of  substances  which  have 
been  constructed  in  a  similar  way,  but  of  which  the 
world  at  large  has  never  heard,  is  equally  conclusive.1 

Still,  we  have  great  reason  to  rejoice  that  this  is  one 
of  the  few  instances  where  purely  theoretical  study  has 
been  unexpectedly  crowned  with  great  practical  results. 

1  The  synthesis  of  indigotine,  the  coloring  principle  of  indigo,  which 
has  since  been  made  by  Baeyer,  of  Munich,  is  as  remarkable  a  result  as 
the  synthesis  of  alizarine,  and  the  process  is  rapidly  becoming  of  equal 
commercial  importance.  Moreover,  the  manufacture  of  the  oil  of  bitter 
almonds  and  of  salicylic  acid,  from  the  products  of  the  distillation  of 
coal-tar,  and  of  vanilline,  the  flavoring  principle  of  vanilla,  from  the  inner 
bark  of  the  pine-tree,  are  already  well-established  industries.  The  oil  of 
bitter  almonds,  which  was  formerly  only  known  as  a  vegetable  product, 
is  the  starting-point,  not  only  in  the  preparation  of  indigotine,  but  also 
of  many  brilliant  dyestuffs ;  and  the  manufacture  of  these  and  of  many 
other  coloring  materials  are  all  most  remarkable  examples  of  the  benefits 
derived  by  the  useful  arts  from  the  results  of  theoretical  science. 


354  SYNTHESIS   OF  ORGANIC  COMPOUNDS. 

Let  us  accept  the  gift  with  gratitude,  and  pay  due  honor 
to  those  through  whose  exertions  it  has  been  received. 
Let  us  remember,  however,  that  it  came  as  a  free  gift, 
and  that  the  result  was  achieved  by  men  who,  with 
single-hearted  zeal,  worked  solely  to  extend  knowledge. 
Forget  not,  then,  to  encourage  those  who  are  devoting 
their  lives  to  the  same  noble  service,  and  have  the 
manly  courage  to  sow  the  seed  whose  harvest  they  can 
never  hope  to  reap.  Honor  those  who  seek  Knowledge 
for  her  own  sake,  and  remember  that  they  are  the  great 
heroes  of  the  world,  who  work  in  faith,  and  leave  the 
result  with  God ! 


LECTUKE  XY. 

THERMO- CHEMISTRY. 

DURING  these  lectures  I  trust  we  have  become  fa- 
miliar with  certain  fundamental  conceptions  of  chemi- 
cal science  :  1.  That  every  substance  is  an  aggregate  of 
similar  particles  called  molecules,  in  which  its  qualities 
inhere.  2.  That  excepting  in  a  few  cases,  where  the 
molecules  are  apparently  indivisible,  every  molecule  is 
a  system  of  minute  bodies  hitherto  undivided,  called 
atoms,  which  are  held  together  by  what  appear  to  be 
polar  forces.  3.  That  the  relations  of  different  mole- 
cules are  determined,  not  only  by  the  nature  of  their 
atoms,  but  also  by  their  structure,  which  may  be  more 
or  less  stable. 

In  studying  the  stability  of  molecules,  we  must  dis- 
tinguish between  stability  of  structure  and  stability  of 
association.  Many  molecules,  like  those  of  coal  or  of 
iron,  which  are  exceedingly  stable  in  structure,  become 
highly  unstable  through  their  association  with  the  oxy- 
gen-molecules of  our  atmosphere.  In  studying  stability 
of  structure,  we  have  only  to  consider  the  tendency  of  a 
substance  to  undergo  spontaneously  chemical  change  un- 
der the  influence  of  concussion,  heat,  light,  electricity, 
or  other  agents ;  while  in  studying  stability  of  associa- 
tion, we  have  to  consider  the  tendencies  to  change  in 


356  THERMO-CHEMISTRY. 

concurrence  with  other  substances,  and  especially  in 
contact  with  the  atmosphere.  Of  course,  in  the  largest 
sense,  the  problem  of  stability  of  association  is  as  broad 
as  the  science  of  chemistry ;  for  all  chemical  changes 
not  spontaneous  are  induced  by  the  natural  or  artificial 
association  of  different  materials  under  varying  condi- 
tions. But,  if  we  limit  our  regards  to  the  stability  of 
natural  substances  at  the  surface  of  the  earth,  we  have 
chiefly  to  consider  them  in  their  association  with  water 
or  air.  As  the  tendency  of  Nature  must  necessarily  be 
to  the  state  of  most  stable  equilibrium,  we  should  ex- 
pect to  find  the  crust  of  the  earth  consisting  of  the 
most  stable  substances  under  the  existing  associations, 
and,  in  fact,  the  rocks,  the  earths,  the  metallic  oxides, 
water,  carbonic  dioxide,  and  nitrogen  gas,  are  among 
the  most  stable  substances  known,  and  their  degree  of 
stability  is  shown  by  the  great  amount  of  energy  wrhich 
is  required  to  decompose  them,  or  to  induce  them  to  en- 
ter into  chemical  changes  with  other  substances.  Still, 
by  various  processes,  we  have  succeeded  in  building  up 
molecules  of  a  more  or  less  unstable  structure,  and  also 
such  as  have  a  very  strong  tendency  to  react  on  water 
or  oxygen  gas,  and  which,  therefore,  are  in  a  more  or 
less  unstable  association  in  the  atmosphere.  Moreover, 
under  the  influence  of  the  sun's  rays,  such  products  are 
constantly  being  formed  in  the  growing  animals  and 
plants,  and  oxygen  gas,  with  its  powerful  affinities,  is 
being  stored  in  the  atmosphere.  Bute  with  all  such  un- 
stable products,  there  is  a  constant  tendency  to  revert  to 
the  stable  condition,  and,  while  the  explosion  of  iodide 
of  nitrogen  or  of  nitro-glycerine  are  striking  examples  of 
this  tendency,  where  the  instability  arises  from  structure, 
the  ordinary  phenomena  of  combustion  are  equally  strik- 
ing examples  where  the  instability  arises  from  association. 


STABLE  AND   UNSTABLE   SUBSTANCES.  357 

Now,  what  we  recognize  at  once  to  be  true  in  these 
conspicuous  illustrations  is  universally  true,  namely, 
that  the  falling  back  from  a  less  stable  to  a  more  stable 
condition  is  always  attended  with  the  evolution  of  heat, 
whether  the  change  results  from  the  spontaneous  de- 
composition of  a  single  complex  compound,  or  from  the 
concurrence  of  several  in  a  chemical  reaction.  But 
here,  as  elsewhere  in  Nature,  action  and  reaction  must 
always  be  equal  and  opposite,  and  hence — even  if  we 
could  not  prove  it  to  be  true  by  the  most  conclusive 
evidence — we  might  confidently  assume  that,  through 
whatever  series  of  chemical  changes  these  unstable  prod- 
ucts may  have  been  formed,  an  equal  amount  of  heat  or 
an  equivalent  amount  of  some  other  mode  of  energy 
must  have,  for  the  time  being,  disappeared.  We  have 
already  caught  a  glimpse  of  this  general  truth  in  study- 
ing the  relations  of  the  grand  phenomena  of  combustion 
to  the  sun.  When  wood  burns,  the  chemical  process 
involves  a  falling  back  from  a  material  unstable  through 
association  with  the  oxygen  of  the  atmosphere  to  the 
stable  products,  carbonic  dioxide  and  water.  The  heat 
evolved  is  the  effect  of  this  fall,  and  this  heat  is  the 
exact  measure  of  the  energy  exerted  by  the  sun  in  the 
growth  of  the  wood.  Most  of  the  products  of  organic 
life,  while  unstable  from  their  close  association  with  the 
atmosphere,  are  equally  unstable  from  their  structure. 
In  the  processes  of  fermentation  and  putrefaction  these 
products  fall  into  more  stable  materials;  and  in  like 
manner  the  heat  evolved  in  these  processes  is  simply 
the  measure  of  the  sun's  energy,  which  disappears  in 
the  production  of  such  substances  as  starch,  albumen, 
or  gluten,  in  the  growing  plant. 

Confining  our  attention  for  a  moment  to  the  less 
complex  condition  of  instability  from  structure,  we 


358  THERMO-CHEMISTRY. 

may  compare  the  structure  of  a  molecule  to  the  struct- 
ure of  a  building.  Like  the  molecules  the  erections  of 
man  present  every  gradation  of  stability,  from  that  of 
the  pyramids  to  that  of  a  lofty  wide-spreading  Gothic 
arch  whose  permanence  depends  on  its  key-stone  and 
buttresses,  which  may  be  compared  to  the  multivalent 
atoms  holding  together  the  parts  of  a  molecule  (com- 
pare page  274).  When  the  key-stone  crumbles  or  the 
buttresses  fail,  the  stones  fall  to  a  more  stable  position  on 
the  surface  of  the  earth,  and  in  this  fall  heat  is  devel- 
oped. In  like  manner,  when  from  any  cause  the  atomic 
clamps  of  the  molecules  are  displaced,  the  atoms  rush 
into  more  stable  combinations  and  heat  is  set  free  as 
before ;  the  polar  attractions  of  the  atoms  thus  produc- 
ing effects  similar  to  the  familiar  phenomena  of  gravi- 
tation. The  parallelism  between  the  two  cases  is  very 
striking,  and  the  well-known  mechanical  principles  in- 
volved in  the  building  and  ruin  of  an  edifice  will  help 
us  to  understand  the  similar  although  much  more  ob- 
scure phenomena  in  chemistry.  As  in  building  an 
edifice  the  same  amount  of  energy  must  have  been  ex- 
pended in  raising  the  stones  that  was  exhibited  in  their 
fall,  so  also  an  amount  of  energy  must  have  been  used 
to  form  the  molecule  equal  to  the  heat  set  free  in  its 
decomposition.  Again,  as  the  same  total  energy  must 
be  used  to  lift  each  granite  block,  whether  raised  in 
successive  stages  or  all  at  once,  so  the  amount  of  en- 
ergy required  to  form  a  molecule  is  the  same,  whether 
it  be  formed  in  a  single  reaction  or  by  a  circuitous 
process ;  and,  on  the  other  hand,  the  amount  of  heat 
evolved  is  the  same,  whether  the  molecule  springs  back 
to  the  most  stable  materials  at  one  bound,  as  in  the 
case  of  nitro-glycerine,  or  in  a  succession  of  stages,  as 
in  the  processes  of  organic  decomposition.  Lastly,  as 


ARCHITECTURAL   COMPARISON.  359 

in  the  fall  of  an  edifice,  the  amount  of  heat  produced  will 
be  proportional  to  the  total  fall,  and  will  be  at  a  maxi- 
mum when  the  stones  have  reached  the  lowest  possible 
position,  so,  also,  in  the  case  of  molecules,  the  amount 
of  heat  evolved  becomes  a  measure  of  the  degree  of 
stability  attained ;  and  as  the  tendency  is  always  to  the 
greatest  stability,  it  is  also  to  those  products  which  will 
determine  the  maximum  evolution  of  heat. 

This  last  result  is  found  not  only  to  be  true  in  the 
case  of  isolated  molecules  of  unstable  structure  which 
we  can  so  perfectly  compare  with  an  edifice,  but  also 
in  the  case  of  associated  molecules  where  the  illustra- 
tion has  not  the  same  obvious  application,  and  thus  we 
arrive  at  the  most  recent  great  generalization  of  chem- 
istry : 

In  all  cases  of  chemical  change,  the  tendency  is  to 
those  products  whose  formation  will  determine  the 
greatest  evolution  of  heat. 

You  will  realize  the  importance  of  this  generaliza- 
tion if  you  reflect  that  it  gives  us  the  means  of  predict- 
ing the  order  and  results  of  any  chemical  change  in- 
volving known  factors,  in  all  cases  where  the  amounts 
of  heat  that  would  be  evolved  in  all  the  probable  com- 
binations of  the  atoms  are  also  known :  for,  in  every 
case,  those  products  will  result  which  will  determine  the 
maximum  evolution  of  heat.  The  ability  to  make  such 
predictions  is  the  highest  aim  of  science.  It  is  the 
boast  of  astronomy  that  it  can  predict  the  occurrence 
of  an  eclipse,  and,  in  some  cases,  even  the  return  of  a 
comet.  It  is  the  boast  of  optics  that  it  did  predict  the 
phenomena  of  conical  refraction ;  and  it  would  be  a  very 
great  triumph  for  chemistry  if  it  could  predict  the  order 
and  products  of  a  chemical  change  under  all  possible 
conditions  of  association  of  materials  or  of  circum- 


360  TIIERMO-CHEMISTRY. 

stances.  The  important  generalization  we  have  stated 
and  illustrated  enables  us  in  part  to  do  this,  and  you 
can  therefore  understand  our  anxiety  to  fulfill  the  con- 
ditions under  which  alone  such  predictions  become  pos- 
sible. Moreover,  the  law  we  have  discovered  explains 
chemical  phenomena  in  the  same  sense  that  the  law  of 
gravitation  explains  celestial  phenomena,  or  the  undu- 
latory  theory  explains  the  phenomena  of  optics,  indeed 
so  far  as  physical  science  can  explain  any  natural  phe- 
nomena. 

In  order,  then,  to  explain  chemical  processes  and 
predict  their  results,  it  has  been  a  chief  object  of  re- 
cent investigation  to  determine  the  heat  of  formation 
of  all  substances,  so  that  we  shall  be  able  to  know  in 
the  case  of  any  association  of  materials  what  products 
will  give  the  greatest  evolution  of  that  form  of  energy. 
But,  obviously,  in  order  to  determine  the  heat  of  forma- 
tion of  substances,  we  must  begin  with  some  materials 
as  our  basis,  and  there  is  one  class  of  substances,  namely, 
the  elementary  substances,  from  which  all  other  sub- 
stances can  be  produced.  It  is  natural,  therefore,  to 
take  the  seventy  or  more  elementary  substances  as  the 
basis  required.  Here,  however,  a  difficulty  arises,  since 
the  molecules  of  some  of  the  elementary  substances  are 
already  in  a  very  stable  condition.  This  is  conspicu- 
ously true  of  the  molecules  of  nitrogen  gas.  This  sub- 
stance, although  elementary,  is  remarkable  for  its  very 
great  inertness.  There  are  only  a  very  few  chemical 
agents  which  will  act  upon  it,  and  it  is  evident  that  the 
two  atoms  of  which  each  of  its  molecules  consists  (N-N) 
are  held  together  with  great  force. 

Could  we  begin  with  dissociated  atoms,  the  prob- 
lem we  have  proposed  would  be  a  much  simpler  one. 
The  union  of  these  atoms  to  form  molecules  would  be 


THE   GREAT   LAW.  361 

in  every  case  attended  with  evolution  of  heat,  and  the 
different  amounts  evolved  would  be  all  positive  quanti- 
ties, and  all  directly  comparable.  But  beginning  with 
molecules  of  elementary  substances,  the  formation  of 
other  molecules  may  or  may  not  be  attended  with  the 
evolution  of  heat,  and  often  heat  is  absorbed.  The 
atoms  of  which  the  molecules  of  the  elementary  sub- 
stances consist  must  be  drawn  apart  before  they  can  form 
new  molecular  groups,  arid  the  resulting  thermal  effect 
will  depend  on  whether  the  amount  of  heat  absorbed  at 
the  parting  is  greater  or  less  than  that  evolved  at  the 
new  union.  If  it  requires  a  larger  amount  of  heat  to 
part  the  atoms  of  the  molecules  of  the  elementary  sub- 
stances than  that  produced  by  the  subsequent  union  of 
the  atoms  to  form  the  molecules  of  the  compounds  re- 
sulting, then  the  total  effect  will  be  an  absorption  of  heat, 
and  the  formation  of  such  compounds  from  elementary 
substances  always  involves  a  loss  of  energy  in  the  form 
of  heat.  This  is  especially  true  of  the  compounds  of 
nitrogen,  for  nitrogen  gas  is  a  more  stable  substance 
than  most  of  the  compounds  containing  this  element. 
In  most  cases,  however,  the  formation  of  a  compound 
body  from  elementary  substances  is  attended  with  the 
evolution  of  heat,  and  thus  we  come  to  classify  chemical 
compounds  into  exothermous  compounds,  whose  heat  of 
formation  is  a  positive  quantity,  and  endothermous  com- 
pounds, whose  heat  of  formation  is  negative. 

It  must  always  be  kept  in  mind,  while  discussing 
this  subject,  that  the  classification  of  chemical  com- 
pounds as  exothermous  and  endothermous  has  refer- 
ence solely  to  the  actual  elementary  substances  out  of 
which  the  compound  may  be  supposed  in  the  last  analy- 
sis to  have  been  formed.  Thus  water  is  an  exother- 
mous body.  Eighteen  grammes  of  water  may  be  re- 


362  T  EERMO-CHEMISTRY. 

garded  as  formed  from  2  grammes  of  hydrogen  gas  and 
16  grammes  of  oxygen  gas,  and  it  has  been  found  by  ex- 
periment that,  in  forming  this  amount  of  liquid  water 
from  the  amounts  of  hydrogen  and  oxygen  gases  just 
stated, 69,000  units  of  heat  are  evolved.  In  like  manner 
sulphuric  acid  is  an  exothermous  compound.  The  sym- 
bol which  represents  its  composition  (H2SO4)  expresses 
the  fact  that  98  grammes  of  the  compound  contain  2 
grammes  of  hydrogen,  32  grammes  of  sulphur,  and  64 

grammes  of  oxygen  : 

H2  =    2 
S  =  32 

O4  =  64 

HaSO4  —  98 

and  although  sulphuric  acid  cannot  be  formed  by  the 
direct  union  of  these  three  elementary  substances,  still 
it  can  be  indirectly  formed  from  them,  and  in  the  last 
analysis  must  be  referred  to  them.  Now,  it  has  been 
found  that  the  ultimate  production  of  liquid  oil  of  vit- 
riol from  roll-brimstone,  hydrogen  gas,  and  oxygen  gas 
(all,  it  must  be  noticed,  definite  substances  which  can  be 
handled  and  weighed),  involves  the  evolution  of  193,000 
units  of  heat  for  every  98  grammes  of  acid  formed. 
Again,  on  the  other  hand,  nitrous  oxide  (N2O,  page  198) 
is  an  endothermous  compound,  which,  although  it  can- 
not be  formed  by  the  direct  union  of  oxygen  and  nitro- 
gen gases,  is  referred  to  them  in  these  problems,  and  it 
has  been  determined  that,  in  passing  from  28  grammes 
of  nitrogen  gas  and  16  grammes  of  oxygen  gas  to  44 
grammes  of  nitrous  oxide  gas  through  the  various  cir- 
cuitous processes  required  for  the  production  of  this  com- 
pound, 18,000  units  of  heat  disappear.  The  heat  of 
formation  of  nitrous  oxide  is  therefore  negative,  and 
is  expressed  as  —  18,000. 


HEAT  OF  FORMATION.  363 

For  convenience  the  heat  of  formation  in  all  cases 
is  referred  to  the  molecular  weight  taken  as  so  many 
grammes,  thus : 

69,000  units  is  the  heat  of  formation  of  18  grammes  H2O. 
193,000     "  a  "  98         "        H2S04 

—18,000     "  "  "  44        "        ]ST2O, 

and  in  every  case  the  quantity  given  is  the  amount  of 
heat  evolved  in  the  assumed  production  of  the  compound 
from  the  amounts  of  the  several  elementary  substances 
indicated  by  the  symbols,  assuming  that  the  molecular 
and  atomic  weights  are  referred  to  the  gramme  as  the 
unit. 

In  thus  taking  the  elementary  substances  as  our  basis 
of  reference,  we  must  further  assume  that  these  sub- 
stances are  in  a  certain  definite  condition — carbon,  for 
example,  as  diamond ;  sulphur  in  its  natural  crystals ;  oxy- 
gen, hydrogen,  and  nitrogen,  in  their  familiar  condition 
of  gas.  Thus,  nothing  is  left  indefinite  about  our  prob- 
lem. The  symbol  of  a  compound  stands  for  a  certain 
molecular  weight,  consisting  of  equally  definite  weights 
of  the  several  atoms  of  which  the  molecule  consists. 
This  molecular  weight  may  be  referred,  of  course,  to 
any  unit  we  please,  and  in  our  problem  of  thermochem- 
istry we  make  the  unit  a  gramme.  In  like  manner  the 
weights  of  the  several  kinds  of  atoms  we  interpret  as  so 
many  grammes  of  the  corresponding  elementary  sub- 
stance in  a  definite  state,  and  the  quantity  of  heat 
evolved,  positive  or  negative,  is  that  which  results  from 
the  passing  of  so  many  grammes  of  the  elementary  sub- 
stances into  so  many  grammes  of  the  compound  by  vari- 
ous processes  usually  more  or  less  circuitous.  Could  we 
begin  with  dissociated  atoms,  we  should  avoid,  as  has 
been  said,  negative  signs,  and  our  calculations  would  be 


364  THERMO-CHEMISTRY. 

less  involved,  but  for  most  purposes  of  comparison  our 
present  data  are  sufficient ;  only,  as  in  algebra,  we  must 
pay  careful  regard  to  the  signs. 

Having  now  fully  stated  the  great  problem  of  ther- 
mo-chemistry  and  its  object,  we  have  next  to  consider 
how  practically  we  can  measure  the  amount  of  heat 
evolved  in  the  formation  of  compound  substances  from 
the  elementary  substances  of  which  they  may  be  re- 
garded as  composed  ;  and  at  the  very  outset  the  ques- 
tion arises,  How  do  we  measure  heat  ?  We  have  been 
speaking  of  quantity  of  heat ;  what  is  meant  by  this 
phrase  ?  We  may  be  assumed  to  understand  what  is 
meant  by  temperature,  the  condition  indicated  by  the 
thermometer ;  but  we  must  be  very  careful  not  to  con- 
found temperature,  which  is  a  condition,  with  quantity 
of  heat,  which,  like  any  other  mode  of  energy,  can  be 
measured  in  conventional  units. 

But  while  the  thermometer  is  no  measure  of  quantity 
of  heat,  it  does  give  an  accurate  indication  of  a  condi- 
tion on  which  such  a  measure  can  be  based.  To  raise 
the  temperature  of  a  definite  quantity  of  a  given  mate' 
rial  from  one  definite  point  to  another  always  requires 
the  same  quantity  of  heat.  Moreover,  while  with  differ- 
ent substances  the  amount  of  heat  required  to  produce 
the  same  rise  of  temperature  varies  very  greatly,  yet 
with  the  same  substance  the  amount  of  heat  required  is 
exactly  proportional  to  the  mass  or  weight  of  the  mate- 
rial. It  is  also  very  closely  although  not  exactly  pro- 
portional to  the  rise  of  temperature  in  thermometric 
degrees,  and  the  variation  from  the  exact  proportion 
is  so  slight  that  it  may  be  neglected,  except  when  the 
increase  of  temperature  is  very  large  or  in  very  refined 
work.  Hence  we  can  use  the  change  of  temperature  of 
a  known  mass  of  any  substance  as  a  measure  of  quantity 


HOW   WE   MEASURE   HEAT.  365 

of  heat.  For  many  reasons  we  have  selected  pure  water 
as  the  standard,  and  the  amount  of  heat  required  to  raise 
the  temperature  of  one  gramme  of  water  from  0°  to 
1°  centigrade  we  call  a  unit  of  heat,  and  we  name  this 
unit  a  calor.  Quantity  of  heat  is  then  measured  by  the 
weight  of  water  and  the  temperature  through  which 
it  is  raised ;  and  the  number  of  units  or  calors  is 
found  in  any  case  by  multiplying  the  weight  of  water 
in  grammes  by  the  number  of  centigrade  degrees  which 
express  the  rise  of  temperature.  Thus,  if  1,200  grammes 
of  water  were  raised  5°  C.,  we  know  that  6,000  units  of 
heat  or  calors  have  entered  the  water. 

We  have  now  a  standard  with  which  we  can  com- 
pare the  quantities  of  heat  required  to  raise  the  tempera- 
ture of  one  gramme  of  other  substances  1°,  and  these 
quantities  are  what  we  call  the  specific  heat  of  the  sev- 
eral substances.  We  have  already,  on  page  148,  given  a 
table  of  the  specific  heats  of  the  elementary  substances, 
and  discussed  the  remarkable  relation  between  these 
specific  heats  and  the  atomic  weight.  Recurring  to  this 
table,  allow  me  again  to  ask  you  to  notice  that  the  num- 
bers in  each  case  give  the  fraction  of  a  unit  of  heat  re- 
quired to  raise  the  temperature  of  one  gramme,  one  kilo- 
gramme, or  one  pound  of  the  substance  1°  according  to 
the  unit  of  weight  we  adopt ;  and  notice  how  much  less  is 
required  for  the  metals  of  which  our  chemical  vessels  are 
made  than  for  water.  In  the  case  of  platinum  it  only 
requires  TjHhr  °f  a  un^  °f  ^iea^  to  raise  tne  temperature 
of  one  gramme  1° ;  and  hence,  assuming  that  the  tem- 
perature of  a  vessel  of  platinum  weighing  200  grammes 
and  holding  1,000  grammes  of  water  were  raised  5°,  the 
platinum  would  only  receive  32  units  of  heat,  while  the 
water  receives  5,000.  This  circumstance  is  of  great  im- 
portance in  measuring  quantities  of  heat ;  for,  of  course, 


366  THERMO-CHEMISTRY. 

the  water  must  be  held  in  some  vessel,  and  with  a  vessel 
of  platinum,  or  even  of  silver  or  brass,  the  effect  of  the 
metal  is  quite  insignificant.  Still,  we  do  not  neglect  this 
effect,  but  reduce  the  metal  to  what  we  call  its  thermal 
equivalent.  Thus,  the  vessel  of  platinum,  weighing 
200  grammes,  would  have  the  same  calorific  value  as 
200  X  0.032  =  6.4  grammes  of  water,  and  this  vessel 
holding  a  kilogramme  of  water  would  be  the  equivalent 
of  1,006.4  grammes  of  water. 

This  further  discussion  of  the  measure  of  heat  and 
of  specific  heat  was  necessary,  in  order  that  we  may 
fully  understand  the  important  applications  we  are  now 
to  make  of  the  principles  involved.  But  heat  is  merely 
the  mode  of  energy,  and,  before  we  pass  to  consider  the 
actual  methods  of  measuring  the  heat  evolved  in  chemi- 
cal action,  let  us  consider  what  is  the  equivalent  of  the 
unit  of  heat  we  have  adopted  in  the  ordinary  measures 
of  mechanical  energy. 

You  all  know  that  mechanical  energy  is  measured  in 
foot-pounds  or  gramme-metres,  according  to  the  units 
assumed ;  that  is,  by  a  certain  weight  lifted  through  a 
certain  distance  against  the  force  of  gravity,  the  prod- 
uct of  the  weight  into  the  distance  lifted  being  in  every 
case  the  measure  of  the  energy.  To  lift  one  gramme  of 
matter  one  metre,  we  must  expend  one  metre-gramme 
of  energy.  The  lifted  gramme,  moreover,  has  this 
amount  of  what  we  call  energy  of  position.  If  it  falls 
freely,  this  energy  becomes  again  active,  and  by  the 
time  it  reaches  the  earth  the  velocity  acquired  is  the 
exact  equivalent  of  one  metre-gramme.  Now,  the  ve- 
locity acquired  by  one  gramme,  and  indeed  by  any  other 
mass  of  matter,  in  falling  freely  from  a  state  of  rest 
through  one  metre,  is  4.427  metres  a  second ;  and  hence 
a  ball  weighing  one  gramme,  and  moving  with  this 


CALORIMETRY.  367 

velocity,  represents  one  metre-gramme  of  energy.  If 
this  velocity  is  in  any  way  arrested,  this  energy  appears 
as  heat.  Assume  that  we  wind  up  the  weight  of  a 
clock  weighing  1,000  grammes.  Through  one  metre 
we  expend  1,000  metre-grammes  of  energy,  and  as  this 
weight  runs  down  this  amount  of  energy  reappears 
in  giving  motion  to  the  trains  of  wheels  ;  but  if,  instead 
of  acting  on  the  clock,  we  cause  the  weight  to  act  on 
some  form  of  friction-brake,  and  so  arrange  the  press- 
ure on  the  brake  that  the  weight,  having  lost  all  its 
velocity,  should  come  to  rest  after  falling  one  metre, 
then,  as  -we  know,  the  thousand  metre-grammes  would 
all  be  converted  into  heat ;  and  if  our  brake  was  im- 
mersed in  water  whose  weight,  as  well  as  that  of  the 
containing  vessel  and  of  all  parts  of  the  machinery  in 
contact  with  the  water,  were  known,  we  could  easily 
calculate  from  the  rise  of  temperature  the  number  of 
units  of  heat  evolved.  Such  measurements  as  these 
have  been  made  with  the  greatest  accuracy ;  and  it  ap- 
pears, both  from  the  experiments  of  Joule,  of  Manches- 
ter, and  from  the  more  recent  experiments  of  Rowland, 
of  Baltimore,  that  one  unit  of  heat  is  the  equivalent  of 
423  metre-grammes,  or  is  represented  by  a  ball  weigh- 
ing one  gramme  moving  with  the  velocity  of  91  metres 
a  second. 

Returning,  now,  to  the  problem  of  measuring  the 
heat  of  chemical  action,  let  me  call  your  attention  to  the 
great  simplicity  of  the  apparatus  which  we  usually  em- 
ploy. The  most  important  part  of  the  apparatus  is  a  cy- 
lindrical vessel  of  platinum  holding  about  1,000  grammes 
of  water,  and  made  as  thin,  and  therefore  as  light,  as 
circumstances  will  permit.  In  this  vessel  the  chemical 
process  is  conducted,  whenever  possible  in  water  as  a 
medium,  and,  if  this  is  not  possible,  then  in  light  vessels 


368  THERMO-CHEMISTRY. 

of  glass  or  other  material  immersed  in  the  water.  In 
every  case  we  have  to  determine  accurately  the  weight 
of  the  water  and  also  the  weight  of  the  vessels,  of  the 
thermometer,  and  of  the  materials  used ;  and  of  these 
last  weights  we  find  the  equivalent  in  water  by  the 
method  described  above.  The  rise  of  temperature, 
which  is  never  allowed  to  exceed  a  few  degrees,  we 
measure  with  very  delicate  thermometers,  graduated  to 
the  -^Q  of  a  degree,  and  enabling  us  to  read  the  temper- 
ature accurately  to  the  y^-  of  a  degree.  The  result  is 
so  many  grammes  of  water,  or  its  equivalent,  raised  so 
many  degrees  in  temperature  by  the  reaction  between 
known  weights  of  material.  In  order  that  we  may  ob- 
serve the  full  heating  effect,  it  is  obviously  important 
that  the  chemical  change  should  be  quickly  finished, 
and  that  during  the  short  time  required  no  heat  should 
escape  from  the  vessel.  To  prevent  the  escape  of  heat, 
we  attempt  to  insulate  the  platinum  cylinder  as  much 
as  possible.  We  stand  it  first  on  non-conducting  sup- 
ports made  of  cork,  in  a  somewhat  larger  cylinder  of 
polished  silver,  which  will  reflect  any  radiated  heat, 
and  in  like  manner  we  set  the  silver  cylinder  in  a  still 
larger  cylindrical  vessel  of  tinned  copper,  covered  on 
the  outside  with  thick  felt,  and  filled  between  the  walls 
writh  water.  In  addition  we  take  care  to  conduct  the 
experiments  in  a  room  whose  temperature  is  as  nearly 
constant  as  possible.  But,  with  all  these  precautions, 
some  heat  will  escape,  and  if  the  experiment  continues 
more  than  a  few  minutes,  we  determine  the  rate  of  cool- 
ing and  make  allowance  for  it.  It  would  be  tedious 
and  unnecessary,  in  this  connection,  to  describe  the 
many  precautions  we  are  obliged  to  take  in  order  to 
secure  accurate  results.  Our  only  object  is  to  describe 
the  process,  so  far  as  is  necessary,  to  make  clear  and 


CALORIMETRY.  369 

real  its  general  principles.  The  details  necessary  for 
actual  work  will  be  found  in  the  well-known  works  of 
Berthelot  and  Thomsen,  the  two  chemists  who  by  their 
investigations  have  almost  alone  developed  this  whole 
subject  of  thermo-chemistry.  In  these  works  will  also 
be  found  descriptions  of  the  exceedingly  ingenions  ap- 
pliances which  have  been  used  for  measuring  the  heat 
of  combustion,  and  of  many  other  chemical  processes 
which  cannot  be  conducted  in  the  presence  of  water. 
Still,  the  general  principle  is  always  the  same,  and  that 
is  to  measure  the  rise  of  temperature  of  a  known  weight 
of  water,  including  the  vessels  or  utensils  used,  reduced 
to  their  equivalent  in  water. 

In  spite  of  all  ingenuity,  however,  the  larger  num- 
ber of  known  chemical  processes  cannot  be  conducted, 
either  with  sufficient  rapidity,  or  under  such  conditions 
as  to  make  it  possible  to  measure  accurately  the  thermal 
effect,  and  it  is  indeed  rarely  the  case  that  we  can  meas- 
ure the  heat  of  formation  of  a  chemical  compound  in 
a  process  of  direct  synthesis  or  direct  analysis.  As  a 
rule,  we  are  obliged  to  pass  from  the  elementary  sub- 
stances to  the  compound,  or  from  the  compound  to  the 
elementary  substances,  through  a  long  series  of  chemical 
reactions  ;  andxin  many  instances,  very  extended  chemi- 
cal knowledge,  as  well  as  great  ingenuity,  have  been  dis- 
played in  contriving  processes  which  will  effect  the  ob- 
ject, and  which,  at  the  same  time,  are  compatible  with 
the  conditions  of  calorimetry.  Indeed,  in  most  cases, 
the  problems  could  not  have  been  solved  were  it  not  for 
certain  general  principles  which  come  to  our  aid,  and 
the  happy  application  of  these  principles  is  the  most 
striking  feature  of  the  ingenious  processes  to  which  we 
have  referred.  These  principles  are  only  special  cases 
under  the  far  more  comprehensive  law  of  the  conserva- 


370  THERMO-CHEMISTRY. 

tion  of  energy,  and  are  so  obvious  that  you  will  recog- 
nize the  truths  as  soon  as  they  are  stated.  The  general 
law,  as  applied  to  thermo-chemistry,  may  be  stated  thus  : 
Whenever  a  system  of  bodies  undergoes  chemical 
or  physical  changes ',  and  passes  into  another  condition, 
whatever  may  have  been  the  nature  or  succession  of  the 
changes,  the  quantity  of  heat  evolved  or  absorbed  d& 
pends  solely  on  the  initial  and  final  conditions  of  the 
system-,  provided  no  effect  has  been  produced  on  bodies 
outside. 

But,  although  this  statement  covers  the  whole  ground, 
yet,  in  the  investigations  of  thermo-chemistry,  the  gen- 
eral principle  presents  itself  under  so  many  unexpected 
and  surprising  phases,  that  it  becomes  important  to  state 
the  following  subordinate  propositions  : 

1.  The  heat  absorbed  in  the  decomposition  of  a  com- 
pound is  equal  to  the  heat  evolved  in  its  formation, 
provided  the  initial  and  the  final  states  are  the  same. 

In  the  case  of  wood,  for  example,  the  products  of 
its  combustion  are  the  food  of  the  growing  plant ;  and 
hence,  since  the  initial  and  final  states  are  the  same,  we 
are  justified  in  the  conclusion  previously  stated  that  the 
heat  absorbed  in  the  process  of  vegetable  growth  is  equal 
to  the  amount  evolved  during  the  burning  of  the  result- 
ing wood. 

2.  The  heat  evolved  in  a  series  of  successive  chemical 
changes  is  equal  to  the  sum  of  the  quantities  which  would 
be  evolved  in  each  separately,  provided  the  final  condi- 
tions are  the  same. 

Hence  it  is  that,  when  a  compound  cannot  be  formed 
by  the  direct  union  of  elementary  substances,  we  can  de- 
termine the  heat  of  formation  by  measuring  the  calorific 
effects  in  the  several  stages  of  the  indirect  processes  by 
which  it  is  prepared. 


INDIRECT  METHODS.  371 

3.  In  any  series  of  chemical  changes,  when  the  ini- 
tial and  final  conditions  are  the  same,  the  total  thermal 
effect  is  the  same,  however  different  the  processes  by  which 
the  results  may  be  reached. 

Hence,  when  the  thermal  effect  of  a  given  chemical 
reaction  cannot  be  directly  measured,  we  can  often  reach 
the  result  indirectly  in  the  following  way :  We  arrange 
two  systems  of  reactions,  both  of  which  begin  with  the 
same  factors  in  the  same  conditions,  and  end  with  the 
same  products  in  the  same  conditions.  In  one  of  these 
series  of  reactions  there  must  be  no  process  whose  ther- 
mal value,  if  not  already  known,  cannot  be  measured 
with  the  calorimeter.  In  the  other  series  the  chemical 
change  whose  thermal  effect  we  are  investigating  enters 
as  an  unknown  term,  the  effect  of  the  other  reactions 
involved  being  known  or  capable  of  measurement  as  in 
the  first  series.  Since  the  total  thermal  effect  in  the 
two  series  must  be  equal,  it  follows,  from  the  principle 
we  are  discussing,  that,  if  we  subtract  the  sum  of  the 
quantities  known  or  measured  in  the  second  series 
from  the  sum  of  those  known  or  measured  in  the  first 
series,  we  shall  have  the  value  of  the  unknown  quan- 
tity. 

4-  The  difference  between  the  quantities  of  heat 
evolved  in  two  series  of  changes,  starting  from  two  dif- 
ferent states  but  ending  in  the  same  final  state,  is  equal 
to  that  evolved  or  absorbed  in  passing  from  one  initial 
condition  to  the  other;  or,  conversely :  The  difference 
between  the  quantities  of  heat  evolved  in  two  series  of 
changes,  starting  from  the  same  initial  condition  but 
ending  in  two  different  states,  is  equal  to  that  which 
would  be  evolved  or  absorbed  in  passing  from  one  of  the 
final  conditions  to  the  other. 

Thus,  although  no  compound  of  hydrogen  and  car- 


372  THERMO-CHEMISTRY. 

bon  can  be  directly  made  or  decomposed  under  such 
conditions  that  we  can  measure  the  calorific  effect,  yet 
in  any  case  the  heat  of  formation  can  be  found  by  meas- 
uring the  heat  of  combustion  of  the  hydrocarbon ;  for, 
as  the  final  states  are  the  same,  whether  we  burn  the 
hydrocarbon  or  an  equivalent  amount  of  diamond  and 
hydrogen  gas,  the  thermal  difference  between  the  hydro- 
carbon on  the  one  hand  and  the  diamond  and  hydrogen 
gas  on  the  other  must  be  equal  to  the  difference  between 
the  heat  of  combustion  of  the  compound  and  the  sum 
of  the  similar  values  for  the  two  combustible  elementary 
substances  which  have  been  acccurately  measured.  So, 
also,  although  the  reaction, 

803  -I-  H20  =  H2S04, 

on  account  of  its  violence  is  not  compatible  with  accu- 
rate thermal  measurements,  using  the  quantities  of  the 
two  factors  which  the  symbols  indicate,  yet  we  can  de- 
termine the  heat  evolved  by  dissolving,  in  one  experi- 
ment, SO3  (sulphuric  oxide),  and  in  another,  H2S04  (oil 
of  vitriol),  in  a  comparatively  large  amount  of  water, 
when  the  difference  in  the  heat  evolved  in  the  two  cases 
will  be  the  quantity  required. 

5.  When  a  compound  gives  up  one  of  its  elements  to 
another  body,  the  heat  evolved  in  the  reaction  is  the  dif- 
ference between  the  heat  of  formation  of  this  compound 
and  that  of  the  resulting  product. 

Thus,  when  chlorine  gas  over  water  is  exposed  to 
the  direct  rays  of  the  sun,  the  water  is  in  part  decom- 
posed, giving  up  its  hydrogen  to  the  chlorine  and  set- 
ting free  oxygen  gas — 

2H2O  +  2C12  =  4H01  +  02 ; 
and   in   this  reaction  the  heat  evolved  is   the  differ- 


INDIRECT   METHODS.  373 

ence  between  the  heat  of  formation  of  2H,,O  and 
4HC1,  without  taking  into  account  the  accompanying 
thermal  effect  which  may  arise  from  solution  or  other- 
wise. 

Theorems  of  this  sort  might  be  very  greatly  mul- 
tiplied, as  they  have  been  by  Berthelot,  in  his  work 
entitled  "  Essai  de  Mecanique  Chimique,  par  M, 
Berthelot"  Paris,  1879 ;  but  those  I  have  stated  will 
give  a  general  idea  of  the  possibilities  of  the  sub- 
ject. 

It  would  be  highly  interesting  in  this  connection  to 
illustrate  the  general  principles  we  have  stated  by  ex- 
amples, and  to  follow  through  some  of  the  wonderfully 
complex  and  ingenious  processes  by  which  results,  ap- 
parently unattainable,  have  been  reached,  such,  for  ex- 
ample, as  the  heat  of  formation  of  peroxide  of  hydrogen 
or  of  nitro-glycerine.  But  such  processes  could  not  be 
made  intelligible  to  a  general  audience  without  devot- 
ing more  time  to  the  subject  than  circumstances  will 
allow,  and  the  chemical  student  will  find  them  all  fully 
detailed  in  the  publications  of  Berthelot  or  Thomson. 
I  must,  therefore,  content  myself  with  explaining  the 
tables  in  which  the  results  have  been  recorded,  and 
showing  how  these  tables  may  be  used. 

A  great  deal  of  work  has  been  done  in  determining 
the  heat  of  formation  of  chemical  compounds,  and  we 
now  know  the  values,  at  least  approximately,  for  almost 
all  the  important  products.  Tables  giving  these  values 
will  be  found  in  the  books  referred  to  above ;  and  a  very 
complete  set,  corrected  to  date,  is  published  every  year  in 
the  "Annuaire,  publie  par  le  Bureau  des  Longitudes  " 
Paris.  We  give  in  the  table  below  a  very  few  of  the 
values  in  the  form  in  which  they  are  usually  pub- 
lished : 


374 


THERMO-CHEMISTRY. 
Heat  of  Formation. 


Constituents  in 

Condition  < 

)f  Product. 

NAME. 

one  Molecule. 

Gas. 

Liquid. 

Solid. 

Solution. 

Water  

H2  +  O 

+  59.4 

+  69.0 

+  70.4 

Nitric  acid  

N  +  O3  +  H 

+  19  6 

+  27  1 

Nitrous  oxide.  .  .. 
Ammonia  

N2  +  0 
H3  +  N 

—18.0 
+  26.7 

-13.6 

+  35.2 

Ammonic  nitrate. 
Sulphuric  acid..  .  . 
Zinc  sulphate.  .  .  . 
Copper  sulphate.. 

N»  +  H4  +  O, 
8  +  O4  +  H2 
Zn  +  S-f  O4 
Cu  +  S  +  O4 

+  193.0 

+  80.7 
+  194.0 
230.0 
182.6 

+  210.8 
248.4 
198.4 

In  expressing  quantities  of  heat  in  the  gramme-units, 
as  defined  above,  the  last  two  figures  do  not,  as  a  rule, 
have  any  significance,  since  they  are  beyond  the  limit  of 
accuracy  of  our  experimental  methods,  and  in  physical 
problems  it  is  more  common  to  use  the  kilogramme- 
unit,  which  is  one  thousand  times  greater.  So  in  the 
above  table,  for  the  sake  of  condensation,  the  heat  of 
combination  is  given  in  each  case  in  kilogramme  units ; 
but  in  chemical  problems  it  is  practically  more  conven- 
ient to  use  the  smaller  unit,  and  therefore  multiply 
the  values,  when  taken  from  the  table,  by  one  thous- 
and. 

After  what  has  been  said  the  interpretation  of  the 
table  will  be  easy,  but,  in  order  that  we  may  have  the 
facts  clearly  before  us,  let  me  so  far  recapitulate  as  to 
state  that  the  first  line  gives  the  information  that,  when 
2  grammes  of  hydrogen  gas  unite  with  16  grammes 
of  oxygen  gas  to  form  18  grammes  of  water,  59,400 
gramme-units  of  heat  are  evolved  when  the  water  re- 
mains as  vapor,  but  69,000  if  condensed  to  a  liquid,  and 
70,400  if  congealed  and  left  as  ice.  In  like  manner  we 
learn  from  the  last  line  that,  when  63.3  grammes  of 
metallic  copper,  32  grammes  of  sulphur  (roll-brimstone), 


ILLUSTRATIONS.  375 

and  64  grammes  of  oxygen  gas  are  converted  by  a  se- 
ries of  chemical  processes  into  159.3  grammes  of  cupric 
sulphate,  182,600  units  of  heat  will  be  evolved  if  the 
salt  is  left  as  an  anhydrous  solid,  and  198,400  units  of 
heat  if  it  is  left  in  solution  in  water. 

Now,  all  this  accumulation  of  data  has,  as  we  have 
seen,  one  great  end  in  view — to  obtain  the  basis  on 
which  we  can  rest  our  predictions  of  the  order  and  ex- 
tent of  chemical  changes.  As  yet,  however,  the  facts 
we  have  so  laboriously  collected  very  imperfectly  fulfill 
the  necessary  condition,  in  the  first  place,  because  they 
are  not  as  yet  either  sufficiently  numerous  or  sufficiently 
accurate ;  and,  in  the  second  place,  because  the  thermal 
relations  are  not  the  sole  conditions  which  determine 
chemical  changes.  Given  the  chemical  reaction,  there 
is  no  question  that  the  tendency  is  to  those  products 
which  will  determine  the  greatest  evolution  of  heat ; 
but  whether  a  reaction  will  take  place  or  not  is  deter- 
mined fully  as  much  by  the  structure  of  the  associated 
molecules  as  by  the  thermal  relations  of  the  possible 
products.  Indeed,  the  analogy  of  our  architectural  illus- 
tration points  to  this  conclusion.  A  block  of  stone 
weighing  a  ton  bears  very  nearly  the  same  relation  to 
the  force  of  gravitation  on  the  top  of  the  spire  of  Stras- 
bui'g  Cathedral  as  on  the  top  of  the  Great  Pyramid, 
and  on  falling  to  the  ground  would  develop  the  same 
amount  of  heat ;  but,  while  in  the  first  case  it  might  be 
dislodged  by  a  well-directed  bomb,  an  earthquake  could 
scarcely  disturb  its  equilibrium  in  the  second  case.  The 
best  we  have  as  yet  been  able  to  do  with  the  great  law 
of  thermo-chemistry  is  to  give  an  intelligible  explana- 
tion of  known  reactions,  and  of  this  important  service 
the  following  reactions  are  good  illustrations : 

When  we  heat  ammonic  nitrate  until  it  melts,  it  suf* 


376  THERMO-CHEMISTRY. 

fers  spontaneous  decomposition,  and  the  sole  products 
are  nitrous  oxide  and  water  in  vapor  (page  197): 

N2H403  =  N30  +  2H20. 
Why  this  change  ?     By  the  table — 

Heat  of  formation  of  NaO  (endothermous  gas)     —  18,000  0. 
"  u        2H30  (exothermous  gas)... .    +  118,800  " 

+  100,800  C. 
Heat  of  formation  of  N2H4O3  (solid) +  80,700   " 

Amount  of  heat  evolved  by  reaction 20,100  C. 

The  amount  of  heat  evolved  by  the  reaction  must  be 
really  a  little  greater  than  this,  for  the  reaction  does  not 
take  place  until  the  salt  is  melted,  and  the  heat  of  for- 
mation of  ammonic  nitrate  is  less  in  the  liquid  than  in 
the  solid  state  by  just  the  amount  which  the  melted 
salt  would  set  free  in  solidifying — that  is,  by  its  latent 
heat.  Although  from  analogy  we  know  that  in  this 
case  the  latent  heat  must  be  very  small,  we  do  not  know 
what  the  value  is,  and  this  is  an  illustration  of  the  in- 
sufficiency of  data  with  which  we  constantly  meet.  But 
the  solid  salt  is  perfectly  stable,  one  of  our  most  com- 
mon laboratory  reagents,  and,  after  studying  the  above 
calculation,  the  question  naturally  arises,  Why  does  not 
this  considerable  evolution  of  heat  determine  the  de- 
composition of  the  salt  in  the  solid  state  ?  Unquestion- 
ably the  freedom  of  molecular  motion  in  the  liquid 
condition  must  greatly  facilitate  the  reaction.  But  it  is 
not  necessary  to  melt  iodide  of  nitrogen  in  order  to  de- 
compose the  compound,  and  the  amount  of  heat  evolved 
by  the  explosion  which  follows  at  the  touch  of  a  feather 
is  but  little,  if  any,  greater  than  in  the  quiet  reaction 
we  are  studying.  The  difference  probably  depends  on 


ILLUSTRATIONS.  377 

molecular  structure,  which  is  very  unstable  in  one  case, 
and  comparatively  stable  in  the  other ;  and  we  can  read- 
ily imagine  that,  while  a  slight  jar,  setting  in  vibration 
the  atoms  in  the  molecules  of  a  mass  of  iodide  of  nitro- 
gen, may  be  sufficient  to  carry  them  beyond  the  limit  of 
equilibrium,  it  may  require  a  comparatively  high  tem- 
perature so  far  to  increase  the  normal  excursions  of  the 
atoms  in  the  molecules  of  ammonic  nitrate  that  they 
will  topple  over. 

The  obvious  distinction  here  suggested  between  the 
energy  required  to  start  a  reaction  and  the  energy  de- 
veloped in  the  reaction  will  be  rendered  intelligible  by 
recurring  again  to  our  architectural  illustration.  The 
energy  required  to  break  away  a  buttress  of  our  Gothic 
arch  has  evidently  very  different  relations  from  the  en- 
ergy developed  in  the  resulting  fall  of  the  structure  to 
the  earth ;  and  it  is  this  last  only  which  is  comparable 
with  the  thermal  effect  of  a  chemical  reaction.  So, 
when  we  pull  the  trigger  of  a  gun  and  determine  the 
explosion  of  the  inclosed  charge,  the  slight  muscular 
force  exerted  bears  no  proportion  to  the  energy  which 
results  from  the  burning  of  the  gunpowder.  We  merely 
touch  a  spring  which  brings  the  atomic  forces  into  ac- 
tion ;  and  the  study  of  this  and  of  thousands  of  other 
similar  phenomena  may  well  make  us  thoughtful,  for 
they  are  constantly  teaching  us  how,  and  how  only, 
man  rules  the  World,  not  through  his  own  feeble  mus- 
cular energy,  but  by  directing  the  forces  of  Nature.  He 
has  learned  to  touch  the  springs  of  action,  which  call 
into  play  forces  before  which  his  own  strength  is  as  noth- 
ing, but  which,  guided  by  his  intelligence,  have  become 
the  servants  of  his  will. 

Let  us  now  study  another  familiar  chemical  process — 
the  production  of  hydrogen  gas  from  the  action  of  zinc 

26 


378  THERMO-CHEMISTRY. 

on  dilute  sulphuric  acid.  The  reaction  is  expressed  by 
the  equation  before  given  : 

Zn  +  (H2SO4  +  Aq.)  =  (ZnSO4  +  Aq)  +  H3. 

Can  our  thermo-chemical  principles  give  any  account 
of  this  process,  and,  more  especially,  can  they  explain 
why  zinc  acts  so  readily  on  dilute  sulphuric  acid, 
while  copper  does  not  ?  In  the  following  scheme  we 
have  taken  from  the  table  (page  374),  first,  the  heat  of 
formation  of  the  factors ;  and,  secondly,  the  heat  of  for- 
mation of  the  products.  From  these  experimental  data 
we  easily  calculate  the  difference  between  the  two.  Of 
course,  as  we  estimate  the  heat  of  formation  from  act- 
ual elementary  substances,  and  not  from  any  theoretical 
atoms,  it  is  evident  that  the  heat  of  formation  of  the 
elementary  substances  themselves,  such  as  zinc,  copper, 
or  hydrogen  gas,  must  be  nothing.  Then  we  have — 

Heat  of  Formation  of  the  Factors. 

Zinc,  Zn 0.000 

Dilute  Sulphuric  Acid,  H2  +  S  +  O4+  Aq 210,800 

210,800 

Heat  of  Formation  of  the  Products. 

Hydrogen  Gas,  H2 0.000 

Solution  of  Zinc  Sulphate,  Zn  +  S  +  O4  +  Aq 248,400 

248,400 
210,800 


Heat  evolved  by  reaction,  that  is,  when  65.2  grammes  of  )  ^  60Q 
zinc  dissolves  in  dilute  sulphuric  acid \ 

Here,  then,  is  an  explanation  of  the  process.  By 
means  of  this  reaction  the  atoms  are  brought  into  so 
much  closer  association  that  37,600  units  of  heat  are 
the  result,  and  this  amount  of  heat  is  the  measure  of 
the  energy  of  the  chemical  action,  just  as  in  the  falling 


ILLUSTRATIONS.  379 

of  the  arch  the  heat  developed  is  the  measure  of  the 
energy  of  the  leveling  force ,  of  gravitation.  But  why 
is  not  a  similar  reaction  possible  with  copper?  Why 
may  we  not  have — 

Cu  +  (H2S04  +  Aq.)  =  H2  +  (CuS04  +  Aq.)? 

So  far  as  the  form  of  statement,  the  reaction  appears 
just  as  probable  as  the  other.  Let  us  consider  the  ther- 
mal relations  as  shown  by  our  table. 

Heat  of  Formation  of  Factors. 

Copper  Cu 0.000 

Dilute  Sulphuric  Acid 210,800 

210,800 

Heat  of  Formation  of  Products. 

Hydrogen  Gas  H2 0.000 

Solution  of  Copper  Sulphate  Cu  +  S  +  O4  +  Aq.  198,400 

198,400 

Evidently,  the  heat  would  be  absorbed,  and  not  evolved, 
by  the  assumed  reaction,  so  that,  on  bringing  metallic 
copper  in  contact  with  dilute  sulphuric  acid,  so  far  from 
there  being  any  tendency  to  this  reaction,  it  would  re- 
quire the  expenditure  of  an  amount  of  energy  repre- 
sented by  12,400  units  of  heat  to  produce  it.  The  re- 
verse reaction,  however,  ought  to  be  readily  obtained, 
and  we  should  expect  that  hydrogen  gas  would  separate 
copper  out  of  a  solution  of  the  sulphate  (reduce  the  salt, 
as  we  usually  term  such  a  process) ;  and  so  it  will  un* 
der  certain  conditions.  But  12,400  units  of  heat  do 
not,  after  all,  represent  a  very  strong  tendency,  and 
there  are  proba,bly  unknown  conditions  of  structure  or 
state  of  aggregation  which  ordinarily  intervene  to  pre- 
vent the  reaction. 


380  THERMO-CHEMISTRY. 

Examples  like  these  might  be  greatly  multiplied,  but 
they  often  would  involve  complex  chemical  relations 
which  cannot  be  intelligently  discussed  in  elementary 
lectures,  and  the  simple  cases  that  have  been  selected  are 
sufficient  to  show  how  far  the  new  principles  which 
have  been  developed  in  the  study  of  thermo-chemistry 
may  be  said  to  explain  chemical  processes. 

These  examples  suggest  a  question  which  you  may 
desire  to  ask,  and  which  I  will  therefore  anticipate.  If 
the  tendency  in  chemical  reactions  is  always  to  those 
products  which  will  determine  the  greatest  evolution 
of  heat,  how  does  it  come  to  pass,  in  any  case,  that  un- 
stable substances  should  be  formed  ?  How  is  it  possible 
to  pass  from  elementary  substances  to  what  we  have 
called  endothermous  compounds  ;  or  from  exothermous 
compounds  to  elementary  substances  ?  How,  in  an  at- 
mosphere of  oxygen  gas,  have  organic  tissues,  beds  of 
coal,  the  useful  metals,  and  other  combustible  substances, 
ever  been  formed  or  produced  ?  In  a  word,  how  is  any 
chemical  reaction  possible  which  involves  an  absorption 
of  heat  ?  Such  processes,  however,  are  constantly  going 
on.  Coal  is  the  remains  of  organic  tissues  which  grew 
in  the  early  geological  ages,  and  similar  tissues  are  now 
growing  under  the  same  sunshine  as  of  old ;  the  useful 
metals  are  readily  extracted  from  their  ores,  and  highly 
unstable  products,  like  nitro-glycerine,  are  easily  obtained 
by  well-known  chemical  reactions.  How  is  this  possible  ? 

This  is  one  of  those  questions  which  it  is  much  easier 
to  ask  than  to  answer.  Indeed,  we  can  not  answer  it 
definitely  in  the  present  condition  of  our  science.  We 
know  that,  whenever  a  chemical  reaction  involves  the 
absorption  of  heat,  the  requisite  amount  of  energy  is 
exerted  by  some  agency  outside  of  the  system  of  bodies 
directly  involved  in  the  chemical  change.  In  a  few 


UNSTABLE  PRODUCTS.  381 

cases,  as  in  the  decomposition  of  water  by  an  electric 
current,  or  the  decomposition  of  carbonic  dioxide  in 
the  green  cells  of  a  plant  by  the  sun's  rays,  we  can  trace 
the  energy  to  its  immediate  source,  although,  as  yet, 
we  know  nothing  of  its  mode  of  action.  But,  in  most 
cases,  all  we  know  is  the  bare  fact  that  heat  is  absorbed 
in  the  reaction,  and  in  most  cases  the  chemical  process 
was  discovered  empirically  long  before  the  thermal  rela- 
tions were  known.  We  infer  that,  as  energy  is  required 
to  raise  the  stones  from  the  earth  and  place  them  in  po- 
sition on  the  Gothic  arch  of  our  illustration,  so  energy 
is  required  to  part  the  atoms  in  the  molecules  of  the  fac- 
tors in  our  reaction,  and  place  them  in  the  new  relations 
which  they  occupy  in  the  products.  But,  while  we  can 
see  the  building  of  the  arch,  we  can  only  follow  the 
building  of  molecules  in  imagination.  Still,  through 
the  structural  formulae,  which  have  been  developed  by 
the  study  of  organic  products,  and  of  which,  in  the 
previous  chapters,  we  have  endeavored  to  give  some 
partial  conception,  we  have  been  able  to  picture  to 
ourselves,  however  imperfectly,  the  manner  in  which 
multivalent  atoms,  acting  as  atomic  clamps,  serve  to 
bind  together,  and  yet  to  keep  apart,  other  atoms  which 
have  a  strong  attraction  for  each  other.  We  can  see 
how,  in  the  molecules  of  ammonic  nitrate — to  recur  to 
a  previous  but  striking  illustration — the  nucleus  N-O-N 
may  keep 

H 

H    I  O 

>N-0-£T<  =  N-O-N          +  H-O-H  +  H-O-H 

H      I  O  Nitrous  Oxide.  Water. 

H 

Ammonic  Nitrate. 

the  atoms  of  hydrogen  and  oxygen  at  the  opposite  poles 
of  the  molecule  apart  from  each  other.  We  can  realize 


382  THERMO-CHEMISTRY. 

how  unstable  such  a  grouping  must  be,  and  that  a  very 
small  excursion  of  the  oxygen  and  hydrogen  atoms 
caused  by  an  increasing  temperature  might  be  sufficient 
to  bring  them  within  the  sphere  of  their  mutual  strong 
attraction,  when  they  would  rush  together,  and  the  re- 
action we  have  written  would  result.  We  can  under- 
stand that,  to  reverse  this  reaction — that  is,  to  draw 
apart  the  oxygen  and  hydrogen  atoms,  and  replace  them 
at  the  poles  of  the  original  nucleus — must  require  the 
great  expenditure  of  energy  which  we  have  already  esti- 
mated. But,  although  we  know  that  just  this  amount 
of  energy  was  expended  in  the  indirect  processes  by 
which  the  salt  was  obtained,  and  can  trace  the  various 
steps,  yet  in  most  cases  we  fail  to  see  where  the  force 
which  did  the  work  was  applied. 

There  are,  however,  certain  general  considerations 
which  may  help  the  imagination  to  form  a  picture  in 
outline  of  the  wonderful  processes  which  are  going  on 
around  us  in  Nature,  and  which  in  part  we  control.  We 
know,  for  example,  that  the  sun's  rays,  acting  through 
the  chlorophyl  of  the  leaf,  have  the  power  to  part  the 
atoms  of  oxygen,  hydrogen,  carbon,  and  nitrogen,  in  the 
molecules  of  water,  carbonic  dioxide,  ammonia,  and  ni- 
trogen gas,  floating  in  the  atmosphere  or  dissolved  in  the 
rain-water,  and  that  from  these  parted  atoms  the  mate- 
rials of  organized  beings  are  formed.  We  know,  also, 
that  the  energy  thus  derived  from  the  sun  reappears 
whenever  the  woody  tissue,  the  starch,  and  the  muscle, 
which  thus  result,  fall  back  again  into  the  more  stable 
materials  from  which  they  sprang.  We  know  further 
that  to  the  latent  energy  of  the  unstable  products  of  the 
sun's  action  may  be  referred,  directly  or  indirectly,  the 
formation  of  all  the  unstable  products  of  the  arts,  such 
as  the  violent  explosives,  the  brilliant  dyes,  and  the  use- 


SUN'S  ENERGY.  383 

ful  metals.  These  may  be  either  intermediate  products 
in  the  falling  back  of  organic  materials,  or  they  may  be 
formed  indirectly  by  the  effect  of  this  fall  in  the  same 
way  that  by  proper  mechanical  appliances  the  fall  of  a 
weight  through  a  short  distance  may  be  made  to  raise 
a  lighter  weight  to  a  proportionally  greater  height,  by 
what  in  mechanics  is  called  the  principle  of  virtual  ve- 
locities. 

The  formation  of  nitrates,  from  which  the  chief  ex- 
plosives are  prepared,  is  the  result  of  the  partial  oxida- 
tion of  organic  matter  (when  under  the  influence  prob- 
ably of  certain  lower  forms  of  organic  life),  and  the  brill- 
iant coal-tar  colors  are  obtained  from  the  products  of  the 
decomposition  of  organic  tissues ;  and  both  these  classes 
of  substances  are,  therefore,  examples  of  intermediate 
products  in  the  falling  back  of  organic  matter,  while 
in  the  smelting  of  metals  from  their  ores  the  production 
of  the  metal  is  an  indirect  result  of  the  burning  of  coal 
— a  comparatively  large  amount  of  coal,  in  falling  back 
into  the  condition  of  carbonic  dioxide,  raising  a  less 
amount  of  stable  metallic  oxide  to  the  unstable  metallic 
state.  It  is  true  that  we  thus  see  only  a  shadowy  outline 
of  these  processes,  but  this  is  all  we  can  see  as  yet,  and 
we  must  wait  until  a  deeper  insight  enables  us  to  fill 
out  the  details. 

In  connection  with  the  phenomena  we  have  been 
studying,  another  interesting  question  arises  :  How  far 
does  the  heat  of  chemical  combination  enable  us  to 
measure  the  relative  attractive  force  between  the  ele- 
mentary atoms  ?  Unfortunately,  our  answer  to  this  ques- 
tion must  be  even  less  satisfactory  than  our  answer  to  the 
last.  In  this  lecture  we  have  constantly  kept  in  view 
the  distinction  between  the  isolated  elementary  atoms 
and  the  molecules  of  the  elementary  substances ;  be- 


384  THERMO-CIIEMISTRY. 

tween  the  atoms  of  oxygen,  for  example,  and  the  mole- 
cules of  oxygen  gas ;  between  the  atoms  of  iodine  and 
either  the  molecules  of  iodine  vapor  or  the  crystalline 
molecules  of  the  same  elementary  substance  in  the  solid 
state ;  and  it  must  never  be  forgotten  that  the  heat  of 
formation  given  in  our  tables  is  the  thermal  effect  pro- 
duced in  passing  from  concrete  elementary  substances 
to  an  equally  concrete  compound  substance ;  and  this 
chemical  change  usually  implies  the  breaking  up  of  old 
molecules  and  the  formation  of  new  molecules,  just  as 
much  as  in  passing  from  one  set  of  compound  substances 
to  another.  Moreover,  such  effects,  complex  as  they 
may  be,  are  usually  still  further  complicated  by  the  cir- 
cumstance that  the  chemical  change  is  constantly  accom- 
panied by  a  change  in  the  state  of  aggregation — a  gas 
is  developed  from  solid  and  liquid  materials,  or  a  solid 
separates  from  a  previous  condition  of  solution ;  and  we 
know  that  these  changes  of  physical  state  are  always 
accompanied  by  marked  thermal  effects  independently 
of  all  chemical  action.  Hence  it  is  very  difficult  so  to 
eliminate  other  causes  as  to  determine  the  effect  of  the 
atomic  forces  alone.  Yet  this  is  the  great  problem  be- 
fore us,  and  if  we  could  only  refer  back  the  heat  of 
formation  of  all  substances  to  the  isolated  atoms — we 
should  greatly  simplify  the  relations  of  our  subject.  We 
could  do  this  if  we  knew  the  heat  of  formation  of  the 
elementary  substances  from  isolated  atoms — what  we 
may  call  the  chemical  potential  of  each  kind  of  atoms 
toward  themselves.  This  is  by  no  means  an  impos- 
sible problem,  and  Professor  Thomsen,  of  Copenhagen, 
has  taken  some  steps  toward  its  solution  ;  but  at  pres- 
ent we  can  only  answer  the  question  we  have  asked 
by  thus  repeating  it  in  a  somewhat  more  definite 
form.  Yet  even  this  is  some  advance,  for  definitely  to 


DISSOCIATION.  385 

ask  a  question  is  already  a  long  step  toward  its  solu- 
tion. 

The  last  question  suggests  still  another  :  Can  the  at- 
oms exist  in  an  isolated  condition  ?  Are  isolated  atoms 
a  possibility,  or  only  a  fiction  of  our  imagination  ?  If 
the  atoms  can  exist  isolated,  then  we  should  expect  that 
the  effect  of  intense  heat  would  be  to  bring  all  mate- 
rials into  this  condition.  We  assume  that  the  union  of 
such  isolated  atoms  is  attended  with  the  liberation  of 
heat ;  and,  if  so,  we  should  naturally  infer  that  the  effect 
of  heat  would  be  to  part  the  atoms  again.  Now,  this  is 
exactly  what  we  find  to  be  true,  so  far  as  our  experiments 
extend,  and  the  effect  of  heat  in  parting  atoms  is  what 
we  technically  call  "  dissociation." 

A  large  number  of  chemical  compounds  can  not  even 
be  melted  without  decomposition.  Long  before  the 
molecules  become  loosed  from  their  crystalline  bonds, 
the  more  active  atoms  break  away  and  seek  new  asso- 
ciations, giving  rise  to  those  volatile  products  which  re- 
sult from  the  various  processes  of  destructive  distillation. 
Other  chemical  compounds  can  be  melted,  but  can  not 
be  volatilized  without  decomposition.  In  some  cases,  as 
with  ammonic  nitrate,  the  decomposition  is  so  funda- 
mental that  the  original  substance  can  not  be  again 
produced  by  the  direct  union  of  the  products ;  while  in 
other  cases  it  is  more  or  less  partial,  and  on  condensing 
the  apparent  vapor  the  original  substance  reappears. 
The  last  is  the  case  with  ammonic  chloride  (sal-ammo- 
niac), which,  when  heated,  yields  a  mixture  of  ammonia 
and  hydrochloric-acid  gases,  but  on  cooling  the  aeriform 


NII4C1 


NII3 


IICl 


products,  the  familiar  white  salt  reappears,  and  the  ex- 
periment  has  the  appearance  of  simple  sublimation.    So, 


386  THEKMO-CHEMISTRY. 

hydrate  of  chloral,  when  heated,  yields  a  mixture  of 
chloral  and  aqueous  vapor,  which  reunite  to  form  the 
well-known  anaesthetic  agent  when  the  mixed  vapors 
are  condensed.  It  is  to  phenomena  of  this  class  that 
the  term  dissociation  is  sometimes  restricted,  and  they 
have  been  very  puzzling  and  have  given  rise  to  a  great 
deal  of  controversy  among  chemists,  because  there  is  no 
visible  evidence  that  a  decomposition  has  taken  place, 
and  our  conclusions  in  regard  to  the  change  are  neces- 
sarily inferential. 

If  Avogadro's  law  universally  holds,  then,  from  the 
principles  we  have  fully  discussed,  it  would  follow  that, 
if  ammonic  chloride  is  decomposed  on  volatilizing  (as 
the  above  reaction  indicates),  the  volume  of  the  mixed 
aeriform  products  would  be  twice  as  great  as  the  normal 
volume  of  the  vapor  of  the  compound,  and  this  is  un- 
questionably true.  But  we  here  assume  the  univer- 
sality of  Avogadro's  law,  that  is,  the  equality  of  all  mo- 
lecular volumes  in  the  state  of  gas  or  vapor,  and  that  is 
the  exact  point  at  issue  in  the  controversies  referred  to. 
In  order  to  substantiate  the  law,  it  is  important  to  show 
that,  in  the  vapor  from  ammonic  chloride,  ammonia  gas 
exists  as  such,  and  hydrochloric-acid  gas  as  such  ;  and 
this  chemists  have  endeavored  to  do  by  showing  that, 
when  the  vapor  is  in  contact  with  the  atmosphere,  the 
two  gases  diffuse  unequally.  Here,  again,  there  is  no 
question  about  the  fact ;  but  the  answer  is,  that  the  same 
force  which  causes  the  unequal  diffusion  also  determines 
the  decomposition  of  the  substance.  There  is,  how- 
ever, one  class  of  evidence  which  seems  to  be  conclusive, 
and  that  is  the  evidence  of  the  spectroscope. 

It  would  be  impracticable  in  these  lectures  to  discuss 
either  the  theory  or  the  use  of  this  spectroscope.  In- 
deed, the  subject  is  so  large  that  it  would  require  an 


EVIDENCE   OF  THE   SPECTROSCOPE.  387 

equally  extended  course  to  treat  it  satisfactorily.  I  must 
assume,  therefore,  that  my  audience  are  familiar  with 
the  general  features  of  the  phenomena  observed  with 
the  spectroscope,  and,  if  any  are  not,  I  hope  that  their 
interest  in  chemical  philosophy  will  lead  them  to  acquire 
the  necessary  knowledge  from  one  of  the  many  popular 
books  which  treat  of  this  wonderful  instrument.  It 
must  be  sufficient  for  the  present  to  say  that  every  sub- 
stance when  heated  in  the  aeriform  condition  to  a  suf- 
ficiently high  temperature  to  render  it  luminous  emits 
a  characteristic  light,  and  that  this  light,  when  examined 
with  the  spectroscope,  shows  certain  colored  bands  oc- 
cupying definite  positions  in  the  field  of  view,  or  hav- 
ing definite  relations  to  each  other ;  and,  further,  that 
these  bands  are  positive  proofs  of  the  presence  of  the 
substance  at  the  luminous  source.  The  association  of 
bands  in  the  field  of  the  spectroscope  is  what  we  call 
technically  the  spectrum  of  the  substance,  and  the  spec- 
tra of  the  elementary  substances  have  been  studied  and 
mapped  with  great  care.  The  spectra  of  only  a  few 
compound  substances  have  ever  been  observed,  simply 
because,  with  a  few  exceptions,  all  chemical  compounds 
are  decomposed  before  they  reach  the  temperature  at 
which  they  become  luminous  ;  but,  whenever  seen,  the 
spectra  of  compound  substances  are  found  to  be  just 
as  characteristic  as  those  of  elementary  substances,  and 
no  relation  has  been  discovered  between  the  spectrum 
of  the  compound  and  the  spectra  of  the  elements  of 
which  it  consists.  The  spectra  of  compound  bodies, 
however,  show  certain  characteristics,  and  when,  with  an 
increasing  temperature,  the  compound  is  decomposed, 
the  change  is  marked  by  an  entire  change  in  the  spec- 
trum, one  set  of  bands  disappearing  and  a  wholly  differ- 
ent set  coming  in  their  place. 


388  THERMO-CHEMISTRY. 

A  very  simple  and  common  mode  of  observing  the 
spectrum  of  a  substance  is  to  melt  a  small  portion 
of  it  to  a  bead  on  a  loop  of  platinum  wire  and  hold 
the  bead  thus  supported  in  the  non-luimnous  flame  of 
of  a  Bunsen  gas-burner.  The  material,  being  volatilized 
by  the  high  temperature,  fills  the  flame  with  its  vapor, 
which,  rendered  luminous  at  this  temperature,  shines 
with  its  peculiar  light.  When  the  heat  of  the  Bunsen 
burner  is  not  sufficient  to  volatilize  the  substance,  we 
employ  for  the  same  purpose  the  heat  of  an  electric 
spark.  ]STowr,  wrhen  in  this  way  we  experiment  on  dif- 
ferent salts  of  the  metal  sodium,  we  obtain  in  each  case 
the  same  simple  spectrum  as  if  we  used  the  metal  alone, 
showing  conclusively  that  in  every  case  the  compound 
must  be  decomposed,  and  that  it  is  the  elementary  sub- 
stance which  radiates  the  peculiar  yellow  light  of  the 
flame.  Take,  for  example,  common  salt,  or  sodic  chlo- 
ride (JTaCl).  A  bead  of  this  substance  held  as  described 
in  the  flame  of  a  Bunsen  burner  soon  fills  the  flame  with 
a  vapor  which  emits  the  characteristic  light  of  sodium, 
and  hence,  although  after  passing  through  the  flame  the 
vapor  may  be  again  condensed  and  the  salt  recovered, 
yet  it  is  obvious  that  while  under  the  heat  of  the  flame 
it  must  have  been  decomposed  ;  and  it  is  reasonable  to 
conclude  that  the  same  was  true  in  the  case  of  ammonic 
chloride  where  such  direct  evidence  was  wanting. 

Besides  the  large  class  of  substances,  all  of  which 
cannot  be  volatilized,  and  many  of  which  can  not  even 
be  melted  without  decomposition,  there  is  another  class 
which,  although  comparatively  limited  in  number,  is  the 
one  on  which  we  are  most  apt  to  dwell  in  our  discus- 
sions of  chemical  philosophy ;  a  class  of  substances  which 
can  readily  be  converted  into  vapor,  and  whose  vapors, 
through  more  or  le^s  wide  limits  of  temperature,  show 


ISOLATED   ATOMS.  389 

the  normal  density  which  the  known  molecular  weight 
of  the  substance  requires.  But  in  most  cases  these  gases 
or  vapors,  when  heated  to  still  higher  temperatures,  suf- 
fer decomposition,  and  our  experiments  indicate  that 
the  tendency  is  to  a  condition  of  isolated  atoms  which, 
in  a  few  cases  at  least,  we  have  reached.  Sometimes 
the  decomposition  is  shown  by  permanent  products,  as 
when  ammonia  gas  passed  through  a  red-hot  porcelain 
tube  yields  a  mixture  of  hydrogen  and  nitrogen  gases ; 
sometimes,  although  recombination  ensues  when  the 
products  cool,  a  permanent  decomposition  may  be  ef- 
fected by  drawing  off  one  of  the  constituents  while  at 
a  high  temperature  by  diffusion  or  otherwise.  Thus 
water,  although  such  a  stable  compound,  is  resolved  into 
a  mixture  of  oxygen  and  hydrogen  gases  at  tempera- 
tures above  1200Q,  and,  if  steam  is  passed  through  a  tube 
of  the  metal  palladium  heated  above  this  point,  the  hy- 
drogen gas  will  diffuse  through  the  walls  of  the  tube, 
while  the  oxygen  thus  left  in  excess  can  be  collected 
when  the  steam  is  condensed  at  the  other  end.  Some- 
times the  decomposition  can  only  be  followed  by  the 
change  of  density,  and  these  are  the  most  interesting 
cases. 

As  the  density  shows,  when  sulphur  boils,  the  vapor 
evolved  consists  of  molecules  formed  each  of  six  sulphur- 
atoms  ;  but,  if  this  vapor  is  heated  above  860°,  the  very 
greatly  changed  density  indicates  that  its  molecules  then 
consist  of  only  two  atoms.  Could  we  measure  the  den- 
sity at  still  higher  temperatures,  we  should  probably 
find  that  the  molecules  would  be  reduced  to  single 
atoms.  This  condition  has  been  reached  with  iodine- 
vapor  at  the  temperature  of  a  blast-furnace,  although 
iodine-vapor  below  700°,  like  sulphur-vapor  above 
860°,  has  molecules  consisting  of  two  atoms.  Then 


390  THERMO-CHEMISTRY. 

the  metals  mercury,  cadmium,  and  zinc,  give  vapors 
whose  molecules,  immediately  above  the  boiling-point, 
consist  of  single  atoms.  All  these  facts  indicate  not 
only  that  the  condition  of  isolated  atoms  is  a  possible 
state  of  matter,  but  also  indicate  that  all  materials  must 
tend  toward  this  condition  in  proportion  as  the  tem- 
perature is  elevated. 

For  studying  the  constitution  of  matter  the  spectro- 
scope has  this  great  advantage,  that  it  can  be  applied  to 
the  most  distant  sources  of  light,  like  the  sun  and  the 
stars ;  and  the  study  of  our  sun  with  this  instrument 
has  been  prosecuted  with  great  zeal,  especially  under 
the  favorable  conditions  presented  by  a  solar  eclipse. 
The  phenomena,  which  the  sun  presents,  are  extremely 
complex,  and  three  distinct  regions  of  activity  on  its 
surface  have  been  distinguished  as  we  go  toward  the 
centre,  and  these  regions  have  been  called  the  corona, 
the  chromosphere,  and  the  photosphere.  In  none  of 
these  regions  do  we  see  any  evidence  of  the  existence 
of  materials  more  complex  than  those  we  call  the  ele- 
mentary substances ;  and  in  the  photosphere — the  focus 
of  most  intense  heat — we  infer  that  the  atoms  of  all  the 
elements  are  isolated,  and  only  held  together  by  the 
immense  gravitating  force  of  the  sun's  mass.  More- 
over, the  red  tongues  of  hydrogen-flame  which  are  such 
conspicuous  objects  in  solar  eclipses,  and  similar  sub- 
ordinate phenomena,  seem  to  result  from  the  throwing 
up  into  the  cooler  chromosphere  of  material  from  the 
seething  mass  below,  when  the  atoms  so  far  unite  as  to 
form  molecules  of  hydrogen  gas,  or  of  other  elementary 
substances.  It  is  true  that  all  this,  though  a  probable 
inference  from  observed  data,  cannot  be  regarded  as 
proved ;  still,  the  probability  of  the  existence  of  iso- 
lated atoms  in  the  sun's  photosphere  is  undoubted,  and 


ISOLATED  ATOMS.  391 

thus  we  are  led  to  recognize  in  our  own  solar  system 
the  existence  of  those  very  ultimate  chemical  elements 
which  we  have  assumed  to  be  the  true  basis  of  our 
science,  and  which,  if  we  accept  the  nebular  hypothesis, 
were  the  original  elements  out  of  which  all  substances 
were  evolved. 

If,  then,  material  consisting  of  isolated  atoms  really 
exists,  this  final  question  is  forced  upon  us :  Can  we 
form  any  probable  inference  in  regard  to  the  nature  or 
origin  of  the  atoms  ?  Of  course,  it  will  be  understood 
that,  in  attempting  to  speculate  on  such  a  question  as 
this,  we  are  not  only  going  beyond  the  region  of  legiti- 
mate inference,  but  also  beyond  the  region  of  legiti- 
mate theorizing,  and  all  that  we  can  say  is,  that  we  are 
speculating  in  the  direction  in  which  science  is  ad- 
vancing, and  we  can  not  give  a  complete  conception  of 
the  scope  of  the  new  chemistry,  unless  we  survey  not 
only  the  ground  which  it  actually  occupies,  but  also 
endeavor  to  catch  a  glimpse  of  the  great  unexplored  re- 
gion beyond. 

A  great  deal  of  attention  has  been  paid  to  studying 
the  relations  of  the  atoms  to  each  other,  but  the  only  in- 
ferences which  we  can  draw  in  regard  to  the  qualities 
of  material  consisting  of  isolated  atoms  are  those  quali- 
ties which  alone  depend  on  the  relative  weights  of  the 
atoms  themselves.  It  was  first  observed  by  Dr.  Prout 
that  the  atomic  weights  were  all  very  closely  even  multi- 
ples of  the  atomic  weight  of  hydrogen,  which  is  usually 
taken  as  the  unit  of  the  system  ;  and  he  advanced  the 
theory  that  the  different  atoms  were  simply  aggregates 
of  hydrogen-atoms.  Subsequent  more  accurate  deter- 
minations of  the  atomic  weights,  while  they  cannot  be 
said  to  have  substantiated  the  hypothesis  of  Prout, 
have  certainly  not  disproved  it,  and  it  is  still  a  remark- 


392  THERMO-CHEMISTRY. 

able  fact  that,  with  a  single  exception  (that  of  chlorine), 
there  is  not  one  of  the  twenty  atomic  weights  that  have 
been  most  accurately  determined,  which  differ  from  an 
even  multiple  value  by  more  than  the  possible  experi- 
mental error.  But,  in  the  present  imperfect  condition 
of  our  knowledge  in  regard  to  the  atomic  weights,  we 
cannot  safely  theorize  on  this  fact.  Another  remark- 
able relation  between  the  atomic  weights  was  discovered 
by  the  Russian  chemist  Mendel  ejeff. 

If  we  arrange  the  elementary  substances  in  the  order 
of  their  atomic  weights,  we  find  that  elements  having 
similar  qualities  recur  at  nearly  fixed  intervals,  and  this 
principle  gives  us  the  basis  for  a  classification  which  ex- 
hibits the  relations  of  the  elementary  substances  to  each 
other  in  a  striking  manner.  Tables  in  which  the  ele- 
mentary substances  are  classified  on  this  system  will  be 
found  in  all  recent  works  on  chemistry.  It  would  be 
interesting  to  discuss  at  length  these  tables,  and  the  rela- 
tions of  the  elements  which  they  exhibit,  and  such  a 
discussion  would  have  an  important  place  in  an  ex- 
tended work  on  chemical  philosophy,  but,  as  it  implies  a 
very  full  knowledge  of  the  relations  of  the  elementary 
substances,  or,  in  other  words,  of  the  whole  field  of 
chemistry,  such  a  discussion  would  be  out  of  place  in  a 
course  of  elementary  lectures. 

That  these  tables  do  not  always  give  prominence  to 
the  most  important  relations  of  the  elementary  sub- 
stances, and  that  they  show  many  arbitrary  features,  is 
to  be  expected  ;  for  we  arrange  the  tables  according  to 
the  weights  of  the  atoms,  and  we  bring  into  comparison 
the  relations  of  elementary  substances  whose  molecules 
are  groups  of  atoms,  and  whose  qualities  must  depend, 
not  only  on  the  grouping  of  the  atoms,  but  also  on  the 
possibilities  of  further  grouping  which  the  atoms  possess. 


RELATIONS  OF  THE  ATOMS.  393 

Moreover,  the  relations  of  the  atoms  cannot  depend  on 
the  mass  alone.  Nevertheless,  such  facts  show  that  the 
mass  must  be  an  important  element  in  determining  their 
chemical  relations. 

The  study  of  the  spectra  of  elementary  substances, 
to  which  we  have  referred  above,  shows  that  the  charac- 
teristic bands  are  frequently  repeated  at  regular  inter- 
vals, exhibiting  a  certain  rhythmic  relation,  and,  more- 
over, that  the  spectra  of  allied  elements  are  to  a  certain 
extent  homologous ;  and  the  only  theory  of  their  pro- 
duction which  we  can  form  leads  us  to  infer  that  the 
light  must  originate  in  corresponding  rhythmic  oscilla- 
tions of  the  atoms  which  constitute  the  luminous  source. 
In  other  words,  some  of  the  relations  of  the  atoms  are 
thus  traced  to  definite  phases  of  oscillatory  motion  ; 
and  thus  we  are  brought  to  this  general  conclusion :  the 
chemical  relations  of  the  atoms  depend  in  the  first  place 
on  mass  and  in  the  second  place  on  their  inherent 
motion,  and  the  ultimate  elements  of  each  immutable 
atom  are  a  definite  mass  and  a  definite  mode  of  motion. 

But,  while  we  recognize  in  our  last  analysis  mass 
and  energy  as  the  only  fundamental  elements  of  Nature, 
let  us  not  forget  that  there  must  be  a  directive  faculty 
by  which  the  atoms  are  arranged  and  controlled.  We 
know  that  man  can  touch  the  springs  of  action,  and 
that  thus  his  intelligence  can,  in  a  limited  measure, 
control  events ;  and  this  prerogative,  which  makes  a 
feeble  creature  the  "  Lord  of  Creation,"  is,  we  believe, 
the  type  of  an  Infinite  Intelligence  "  whose  presence 
glows  in  all  within,  around  us,  and  above." 


INDEX. 


THE  numbers  of  this  index  refer  to  pages.  Attention  is  called 
to  the  lists  of  experiments,  graphic  symbols,  reactions,  and  tables 
given  under  these  several  headings. 


Acetic  acid,  306,  308. 

Acetic  ether,  327,  328;  isomeric 
with  butyric  acid,  324. 

Acetone,  339. 

Acctyl,  328. 

Acids,  273,  279,  287,  292,  317; 
nomenclature  of,  188. 

Acids  and  alkalies,  280,  282,  290 ; 
differences,  296,  298. 

Aggregation,  states  of,  6. 

Alchemy,  121. 

Alcohols,  153,  340. 

Aldehydes,  339. 

Alizarine,  350. 

Alkali,  273,  277,  279  (see  also  Acids 
and  Alkalies). 

Alum,  potassic,  274  ;  ammonic,  315. 

Alumiuic  oxide,  333. 

Aluminum,  Action  on  potassic  hy- 
drate, 290. 

Amidogen,  338. 

Ammonia  gas,  206,  268  ;  heat  of 
formation,  374. 

Ammonic  chloride,  268 ;  nitrate, 
197;  heat  of  formation,  374; 
decomposition  of,  376. 

Ampere's  law,  5. 

Analysis,  110,  138,  191;  of  acetic 
ether,  325 ;  of  alcohol,  153 ;  of 
butyric  acid,  325  ;  of  nitric  acid, 
283;  of  water,  140,  154;  of  salt 
and  sugar,  140. 

Andalusite,  315. 

Anhydride,  313. 


Aniline,  346. 
Anthracene,  350. 
Anthraquinone,  350. 
Anticipations  in  science,  3. 
Aristotle,  113,  255. 
Arithmetic,  chemical,  166,  195. 
Artiads  and  perissads,  272. 
Atomic  bonds,  265;  clamps,  274; 

theory,  117. 

Atomicity  of  hydrates,  309. 
Atoms,   28,   151;   specific  heat  of, 

149  ;  polarity  of,  298 ;  weight  of, 

128,  133,  140,  146 ;  relations  of, 

391 ;  isolated,  385. 
Avogadro's  law,  5,  29,  62,  66. 

Barometer,  31. 

Bases,  317. 

Basic,  definition  of,  291. 

Beauxite,  315. 

Becker  and  Stahl,  2;  5. 

Benzol,  345. 

Berzelius,  295,  317. 

Binary    compounds,    nomenclature 

of,  187. 

Bonds,  atomic,  265. 
Boric  acid,  311. 
Boyle's  law,  33. 
Bunsen's  lamp,  229. 
Burning  (see  Combustion). 
Butyric  acid,  308,  324,  329. 

Calcic  hydrate,  273,  310;  oxalate, 
311;  oxide  (see  Lime)  ;  sulphate, 
274. 


INDEX. 


395 


Calcium,  176. 

Calorimetry,  368. 

Galore,  365. 

Candle,  228,  230. 

Carbolic  acid,  346. 

Carbon,  171,  174,  333;  atomic 
weight,  143 ;  radicals,  334,  335  ; 
quantivalence,  272. 

Carbonic  dioxide.  159,  179,  211, 
226 ;  dioxide  action  on  lime-wa- 
ter, 177  ;  dioxide  decomposed  by 
plants,  172;  dioxide  decomposed 
by  sodium,  169. 

Carbonic  oxide,  304. 

Chalk,  decomposed  by  acids,  183; 
decomposed  by  heat,  182  ;  forma- 
tion, 178;  solution,  180. 

Changes,  chemical  and  physical,  109. 

Charcoal,  burning  of,  224,  239. 

Charles's  law,  43,  62. 

Chemical  changes,  109,  191 ;  com- 
pounds, 110,  114,  122. 

Chlorine,  atomic  weight,  141 ;  gas 
burns  tinsel,  208. 

Chrysoberyl,  315. 

Chrysolite,  314. 

Coal,  burning  of,  226 ;  energy  stored 
in,  227. 

Cohesion,  63. 

Combining  proportions,  123. 

Combustibles,  210. 

Combustion,  210-256 ;  of  charcoal, 
224,  225,  239;  of  hydrogen,  115, 
217,  220;  of  phosphorus,  210, 
214;  of  slow-match  in  oxygen, 
105  ;  of  sulphur  in  nitrous  oxide, 
199 ;  of  sulphur  in  oxygen  gas, 
198 ;  of  tinsel  in  chlorine  gas, 
208;  of  watch-spring  in  oxygen, 
105  ;  history  of  theory,  254. 

Compound  blow-pipe,  220. 

Compound  radicals,  293. 

Compounds  (see  Chemical  Com- 
pounds) ;  not  mixtures,  1 22. 

Conservation  of  energy,  application 
of,  369  ;  of  mass,  90,  92. 

Corundum,  315. 

Cream-of-tartar,  162. 

Crith,  69,  72. 

Crystallization  of  ice,  53  ;  of  sal- 
ammoniac,  51 ;  of  urea,  52. 

Crystals,  effects  on  polarized  light, 
55-61. 


Cupric  sulphate,  heat  of  formation, 
374. 

Cuprous  oxide,  333  ;  reduced  by  hy- 
drogen, 90. 

Cyanic  ether  and  cyanetholine,  332. 

Dalton's  atomic  theory,  124. 

Decomposition,  comparative  ease  of, 
205. 

Definite  proportions,  law  of,  93,  94, 
122. 

Density,  70. 

Density  of  vapors,  78,  284. 

Design  in  Nature,  234. 

Diaspore,  315. 

Diatomic  hydrates,  310. 

Differentiation,  a  method  of  inves- 
tigation, 222. 

Dihydro-sodic  phosphate,  311. 

Dipotassic  oxalate,  310. 

Disodic  sulphate,  310. 

Dissociated  atoms,  360. 

Dissociation,  spectroscopic  evidence 
of,  388. 

Dissociation,  386. 

Divisibility  of  matter,  27. 

Dualistic  theory,  295,  29t>. 

Dumas's  method  for  vapor  density, 
80. 

Electrical  polarity,  300. 

Electrolysis,  295. 

Elementary  substances,  125-129  ; 
table  of,  128. 

Endothermous  compounds,  361  ; 
how  obtained,  380. 

Energy  from  burning,  213-227  ; 
from  the  sun,  235  ;  indestructi- 
ble, 235  ;  required  to  decompose 
water,  114. 

Ether  of  space,  14. 

Ethyl,  828. 

Expansion  by  heat,  of  gases,  11 ;  of 
liquids,  10. 

Exothermous  compounds,  361. 

Experiments  :  aluminum  and  potas- 
sic  hydrate,  291  ;  ammonia  and 
hydrochloric-acid  gas,  206  ;  bands 
on  soap-film,  23 ;  baric  chloride 
and  argentic  nitrate,  260 ;  beer 
shown  to  yield  carbonic  dioxide, 
89  ;  burning  charcoal,  224  ;  burn- 
ing charcoal-powder,  225 ;  burn- 


396 


INDEX. 


ing  hydrogen  gas,  104  ;  burning 
iron,  126  ;  burning  phosphorus 
in  air,  210  ;  burning  phosphorus 
in  oxygen,  214  ;  burning  .watch- 
spring.  105  ;  calcining  chalk,  182  ; 
chalk  and  acid,  183  ;  chlorine  gas 
and  tinsel,  208  ;  coloring  power 
of  aniline  dyes,  347  ;  compound 
blow-pipe,  115;  crystallization  of 
sal-am  in  oniac,  51  ;  crystallization 
of  urea,  52 ;  cupric  oxide  reduced 
by  hydrogen,  90 ;  cupric  sulphate 
and  iron,  258 ;  decomposition  of 
sugar,  101  ;  decomposition  of  wa- 
ter, 103,  106  ;  density  of  vapors. 
79,  82  ;  expansion  of  liquids  by 
heat,  1 0 ;  explosion  of  iodide  of 
nitrogen,  203  ;  explosion  of  hy- 
drogen and  oxygen,  115;  forma- 
tion of  vapors,  9  ;  globular  form 
of  liquids,  50;  gunpowder  burnt 
in  vacua,  240  ,  gunpowder  burnt 
in  air,  240  ;  ice-flowers,  53  ;  iodine 
and  phosphorus,  209  ;  iron  and 
hydrochloric  acid,  288  ;  iron  and 
sulphur,  119;  lead  tree,  257; 
lime-water  and  carbonic  dioxide, 
177  ;  magnetic  curves,  60 ;  Ma- 
riotte's  law,  33,  62  ;  nitric  oxide 
and  oxygen  gas,  206  ;  with  polar- 
ized light,  55-61  ;  Pharaoh's  ser- 
pent, 124  ;  potassic  hydrate  and 
nitric  acid,  282  ;  potassium  and 
water,  281  ;  preparation  of  ni- 
trous oxide,  197  ;  preparation  of 
oxygen  gas,  193  ;  products  of  com- 
bustion weigh  more  than  the  can- 
dle, 231  ;  silver-tree,  259  ;  slak- 
ing of  lime,  177  ;  sodic  carbonate 
and  cream-of -tartar,  162  :  sodic 
carbonate  and  muriatic  acid,  157  ; 
sodic  chloride  and  argentic  ni- 
trate, 260  ;  sodic  silicate  and  mu- 
riatic acid,  314  :  sodium  and  car- 
bonic dioxide,  169;  sodium  and 
water,  276 ;  sulphur  burnt  in  ni- 
trous oxide,  199;  sulphuric  acid 
and  zinc,  288  ;  sulphuric  acid  and 
zinc  oxide,  289  ;  synthesis  of  for- 
mic acid,  304  ;  variations  of  quan- 
tivalence,  '270  ;  weight  of  car- 
bonic dioxide,  158. 
Factors  and  products,  87. 


Feldspar,  316. 

Ferric  chloride,  333. 

Filtering,  177. 

Flame,  217;  how  colored,  220,277, 
281  ;  light  of,  228;  of  wood  and 
coal,  230. 

Formic  acid,  304,  308. 

French  system  of  weights  and  meas- 
ures, 69. 

Fuel,  constituents  of,  227  ;  energy 
of,  232;  products  harmless,  232. 

Garnet,  316. 

Gas,  cause  of  its  tension,  38  ;  char- 
acteristics of  a,  30. 

Gas,  illuminating,  228. 

Gas  volumes,  how  represented,  207. 

Gay-Lussac's  law,  67,  95. 

Gibbsite,  315. 

Glass  not  absolutely  homogeneous, 
13  ;  size  of  molecules,  20. 

Glyceric  acid,  342. 

Glycerine,  242,  342. 

Gold,  variations  of  quantivalence, 
271. 

Graebe,  synthesis  of  alizarine,  351. 

Gramme,  69. 

Graphic  symbols,  275 ;  acetone,  339 ; 
acetic  ether,  328 ;  acetyl,  328 ; 
alizarine,  351  ;  aluminic  oxide, 
333  ;  amide  gen,  338  ;  ammonia 
alum,  315;  ammonia  gas,  268; 
ammonic  chloride,  268  ;  ammoni- 
um, 294;  andalusite,  315  ;  ani- 
line, 346  ;  anthracene,  350  ;  an- 
thraquinone,  350 ;  beauxite,  315  ; 
benzol,  345  ;  butyric  acid,  329  ; 
calcic  hydrate,  273 ;  calcic  sul- 
phate, 274  ;  carbon  radicals,  334- 
338  ;  corundum,  315  ;  cliryso- 
beryl,  315;  chrysolite,  314;  cu- 
prous oxide,  338 ;  cyanogen,  294  ; 
diasporc,  315;  ethyl,  294,  328; 
feldspar,  316;  ferric  cMoride, 
333  ;  formic  acid,  305  ;  fluorides 
of  manganese,  209  ;  garnet,  316  ; 
gibbsite,  315  ;  glyceric  acid,  342  ; 
glycerine,  342  ;  hydrochloric  acid, 
303  ;  hydroxyl,  "338  ;  hypochlo- 
rous  acid,  303  ;  chlorides  of  iron, 
270;  lactic  acid,  342;  methyl, 
292  ;  mercurous  chloride,  333  ; 
naphthaline,  349 ;  nitric  acid, 


INDEX. 


397 


299 ;    nitro-benzol,    345 ;    nitro- 
toluol,  345  ;   nitre-glycerine,  343  ; 
nitryl,  333;  phenol,  348;   phos- 
phorous chloride,  268  ;  purpurine, 
351  ;     pyradine,    348 ;    triplum- 
bic  hydrate,  273  ;  potassic  alumi- 
nic   sulphate,   274;  potassic   hy- 
drate, 298  ;  propionic  acid,  341 ;  I 
propylic  aldehyde,  339 ;  propylic  | 
glycol,    842 ;    propylic    hydride, 
338 ;  pyruvic  acid,   341 ;    quino- 
line,  343  ;  rosaniline,  346  ;  silicic  | 
hydrates,    313 ;    tartronic     acid,  | 
342  ;  toluidine,  346  ;  toluol,  345  ;  i 
valeric  acid,  331  ;    wollastonite,  | 
314. 

Gunpowder,    238 ;  energy  exerted,  ; 
241  ;    products    of    combustion, 
240. 

Hare's  compound  blow-pipe,  115. 

Heat,  measurement  of,  364  ;  nature 
of,  39-46  ;  developed  by  burning, 
213  ;  whenever  atoms  unite,  209 ; 
of  formation,  360  ;  of  reaction — 
decomposition  of  ammonic  ni- 
trate, 376 ;  zinc  and  sulphuric 
acid,  378  ;  copper  and  sulphuric 
acid,  379. 

Hcxatonic  hydrates,  315. 

Hof  mann's  method  for  vapor  den-  j 
sity,  82. 

Homologues,   331  ;  series   of,   308,  | 
339,  340. 

Hydrates,  alkaline  and  acid,  291 ; 
atomicity  of,  309 ;  definition  of,  I 
291,  309;  instability  of,  when 
complex,  312  ;  nomenclature  of, 
189 ;  yield  water  when  heated, 
312. 

Hydrides  of  methyl,  ethyl,  propyl, 
etc.,  339. 

Hydrochloric  acid,  303  ;  action  on 
iron  nails,  288  ;  action  on  sodic 
carbonate,  157 ;  combines  with 
ammonia,  206 ;  neutralizes  alka- 

.     lies,  280. 

Hydrodisodic  phosphate,  311. 

Hydrogen,  atomic  weight  of,  144. 

Hydrogen  gas,  105  ;  burning  of,  217- 
222  ;  preparation  of,  288 ;  syn- 
thesis of  water,  218. 

Hydropotassic  oxalate,  310. 


Hydrosodic  sulphate,  310. 
Hydroxyl,  309,  338. 
Hypochlorous  acid,  303. 

Ice,  crystalline  structure  of,  53. 
Ignition,  point  of,  212. 
Imponderables,  113. 
Intelligence  in  Nature,  234,  236. 
Iodide  of  nitrogen,  203. 
Iodine,  203,  209. 
Iron  chlorides,  270. 
Iron,  variations   of   quantivalence, 

269. 

Isomerism,  324. 
Isopropylic  alcohol,  340. 

Kekule  benzol  theory,  344. 
Kerosene,  339. 
Ketones,  339. 

Lactic  acid,  342. 

Lamp,  a  gas-factory,  228. 

Lavoisier,  254,  316. 

Law  of  Ampere,  5  ;  Avogadro,  5, 
48,  62  ;  Boyle,  33 ;  Charles,  43- 
48,  62 ;  definite  proportions,  93, 
94,  122;  Gay-Lussac,  67,  95; 
Mariotte,  33,  62  ;  maximum  ef- 
fect, 359;  multiple  proportions, 
131;  Newton,  111. 

Licbig,  293. 

Light,  when  manifested,  214,  222 ; 
dimensions  of  waves,  16  ;  disper- 
sion of,  19;  polarized,  54  ;  wave 
theory,  14. 

Lime,  action  on  water,  177;  com- 
position of,  176. 

Lime-kiln,  183. 

Limestones,  how  formed,  181. 

Lime-water,  177. 

Liquids,  characteristics  of,  48;  glob- 
ular form  of,  50. 

Litmus-paper,  182. 

Luminous  planes,  228. 

Madder-dye,  351. 

Magnesic  hydrate,  309 ;  sulphate, 
311 ;  magnetic  curves,  60  ;  polari- 
ty, 298. 

Manganese  fluorides,  269  ;  varia- 
tions of  quantivalence,  269,  271, 

Mariotte's  law,  33,  62. 


398 


INDEX. 


Matter,  relations  to  space,  12 ;  in- 
destructible, 160. 

Maximum  effect,  law  of,  359. 

Maxwell,  "  theory  of  heat,"  46;  "  on 
molecules,"  28. 

Measures  and  weights,  French  sys- 
tem, 69. 

Mechanical  equivalent  of  heat,  367 

Mendelejeff's  classification,  392. 

Mercurous  chlorides,  333. 

Metathesis,  191. 

Metathetical  reactions,  275. 

Metre,  69. 

Microcrith,  75,  136. 

Mixture,  distinguished  from  a  chemi- 
cal compound,  122. 

Molecular  structure,  273,  358. 

Molecules,  6,27,  29,  37,  152. 

Molecules,  building  of,  381 ;  chemi- 
cal definition  of,  99, 1(;0 ;  physical 
definition,  98 ;  distinguished  from 
atoms,  134  ;  how  divided,  100- 
103,  107  ;  of  elementary  sub- 
stances, 135-142  ;  their  integrity 
depends  on  what,  274 ;  size  of, 
20,  26  ;  structure  of,  247,  266,  272, 
275,  302,  358  -,  weight  of,  68,  73, 
85. 

Monatomic  hydrates,  309. 

Multiple  proportions,  law  of,  131. 

Multivalence,  261,  273. 

Naphthaline,  349. 

Naphthas,  339. 

Nature,  her  manifestations,  236. 

Newton,  Sir  Isaac,  111. 

Nitrate  of  zinc,  3 1 8. 

Nitric  acid,  282,  299,  303 ;  symbol 
determined,  282  ;  heat  of  forma- 
tion of,  374. 

Nitro-benzol  and  nitro-toluol,  345. 

Nitrogen,  compounds  with  oxygen, 
132  ;  influence  on  combustion, 
210  ;  molecular  stability  of,  360 ; 
variations  of  quantivalence,  268. 

Nitre-glycerine,  242,  343 ;  experi- 
ment at  Newport,  244 ;  molecular 
structure,  248,  343  ;  theory  of  ac- 
tion, 246-254. 

Nitrous  oxide,  197, 198 ;  analysis  of, 
2'>0 ;  heat  of  formation,  362,  363, 
374. 

Nitryl,  338. 


Nobert's  bands,  17. 

Ores,  smelted  by  solar  energy,  235. 

Organic  compounds,  322  ;  instabil- 
ity of,  357. 

Oxalic  acid,  310. 

Oxides,  acid  and  basic,  317;  no- 
menclature of,  186. 

Oxygen,  atomic  weight  of,  141 ; 
chemical  centre  of  Nature,  317 ; 
relations  to  the  dualistic  theory, 
317. 

Oxygen  gas,  105 ;  relations  to  com- 
bustibles, 210-256 ;  preparation 
of,  193. 

Ferissads  and  artiads,  272. 

Periodic  law,  392. 

Phenol,  348. 

Phlogiston  theory,  112,113,  255. 

Phosgene  gas,  304. 

Phosphorous    acid,  311  ;    chloride, 

268  ;  oxide,  212. 
Phosphorous  chloride,  268. 
Phosphorus,    combustion    of,    210, 

214  ;  variation  of  quantivalence, 

268,  271. 
Physical  changes,    definition,   1 09 ; 

processes      distinguished      from 

chemical,  86. 
Plants  decompose  carbonic  dioxide, 

172. 

Pneumatic  trough,  183. 
Polarity  of  atoms,  298. 
Polarized  light,  51-61. 
Potassic  chlorate,  crystals  of,  197; 

used  for  making  oxygen  gas,  193, 

194;  burning  sugar,  237. 
Potassic  chloride,  crystals  of,  197. 
Potassic  hydrate,   281,  298;  acted 

on  by  aluminum,  291. 
Potassic   nitrate    (saltpetre),    239, 

243,  282. 

Potassium  and  water,  281. 
Potential,  chemical,  384. 
Prediction  of  chemical  changes,  359. 
Projectile  agents,  246. 
Propionic  acid,  308,  341. 
Proportional  numbers,  131 ;  old  sys- 
tem, 156. 
Propylic    alcohol,    340;    aldehyde^ 

339  ;  glycol,  342  ;  hydride,  338. 
Prout's  hypothesis,  391. 


INDEX. 


399 


Pseudo-alcohols,  340. 
Purpurine,  351. 
Pyridine,  348. 
Pyruvic  acid,  341. 

Quantitative  analysis,  139. 

Quantivalence,  262-275 ;  distinctive 
feature  of  the  new  chemistry, 
272 ;  how  far  fixed,  270 ;  vari- 
ations of,  268-272. 

Quinoline,  348. 

Radicals,  simple  and  compound, 
293 ;  consisting  of  carbon-atoms, 
334  ;  metals  and  metalloids,  295  ; 
electro-positive  and  electro-nega- 
tive, 295  ;  serial  relations,  296. 

Reactions,  analytical,  193  ;  syntheti- 
cal, 205;  metathetical,  275;  de- 
scribe results  of  experiments, 
181  ;  expressed  by  symbols,  160; 
indicate  structure,  275,  326  ;  nu- 
merical values  calculated,  166, 
195 ;  acetic  ether  and  potassic 
hydrate,  327 ;  ammonia  and  hy- 
drochloric acid,  206 ;  ammonic 
nitrate  when  heated,  197  ;  butyric 
acid  and  potassic  hydrate,  328 ; 
carbonic  dioxide  and  sodium,  1 69, 
174 ;  carbonic  dioxide  and  sun- 
light, 174;  carbonic  oxide  and 
chlorine  gas,  304  ;  carbonic  oxide 
and  oxygen  gas.  304  ;  chalk  when 
calcined,  181  ;  chalk  and  hydro- 
chloric acid,  183;  coal  and  oxy- 
gen, 226 ;  metallic  copper  and 
chlorine  gas,  208 ;  hydrogen  and 
oxygen,  219 ;  hydrochloric  acid 
and  iron,  288 ;  iodide  of  nitrogen 
when  exploded,  204 ;  lime  and 
water,  177 ;  lime-water  and  car- 
bonic dioxide,  178  ;  magnesium 
and  water,  309 ;  nitric  oxide  and 
oxygen  gas,  207;  potassic  chlo- 
rate when  heated,  194;  potassic 
hydrate  and  aluminum,  291 ;  po- 
tassic hydrate  and  nitric  acid, 
286  ;  potassium  and  water,  281 ; 
sodic  carbonate  and  cream-of-tar- 
tar,  163;  sodic  carbonate  and 
hydrochloric  acid,  160,  163,  165, 
166 ;  sodic  hydrate  and  hydro- 
chloric acid,  280;  sodium  and 


water,  278 ;  sulphuric  acid  and 
zmc,  288 ;  sulphuric  acid  and  zinc 
oxide,  289. 

"  Religion  and  Chemistry  " — refer- 
ence, 234. 

Rochelle  salts  formed  in  bread, 
164. 

Rocks,  cinders  of  a  primeval  fire, 
234. 

Ro  aniline,  346. 

Rules  of  chemical  arithmetic,  167. 

Sal-ammoniac,  crystallization  of,  51. 

Salts,  definition,  319;  nomenclature 
of,  189. 

Science,  its  method  illustrated,  223. 

Series  of  homologues,  80&,  331,  339, 
340 ;  of  volatile  acids,  308. 

Silicic  hydrates,  311-313. 

Slaking  of  lime,  177. 

Snow-flakes,  54. 

Soap-bubbles,  21. 

Soap-film,  effect  of  light  on,  22 ; 
thickness  of,  24. 

Soda,  caustic,  277. 

Soda-water,  179. 

Sodic  carbonate  and  muriatic  acid, 
157;  hydrate,  278. 

Sodium,  action  on  water,  276 ;  va- 
por colors  flame,  277. 

Solar  constitution,  spectroscopic  evi- 
dence, 390. 

Solids,  characteristics  of ,  51 ;  struct- 
ure illustrated,  51-61. 

Specific  gravity,  distinguished  from 
density,  70  ;  of  liquids  and  solids, 
71 ;  of  gases  and  vapors,  73,  79, 
82. 

Specific  heat,  365  ;  of  elementary 
substances,  147. 

Spectroscope,  387. 

Spectroscopic  analysis,  basis  of,  220, 
277,  281. 

Spectrum,  solar,  17. 

Stahl,  255. 

Stability,  degree  of,  356 ;  of  asso- 
ciation, 355  ;  of  structure,  355. 

Stable  and  unstable  compounds, 
thermal  relations  of,  209. 

Structure  of  molecules,  247,  266  ; 
determines  qualities,  324 ;  shown 
by  reactions,  326  (see  Molecular 
Structures). 


400 


INDEX. 


Substances  defined  by  their  mole- 
cules, 100;  elementary,  125. 

Sugar,  burnt  by  potassic  chlorate, 
237;  decomposed  by  heat,  101; 
decomposed  by  sulphuric  acid, 
101. 

Sulphate  of  lime,  318. 

Sulphur,  condition  of  vapor,  389. 

Sulphuric  acid,  action  on  zinc,  288  ; 
action  on  zinc  oxide,  289 ;  graphic 
symbol,  310 ;  heat  of  formation 
of,  362,  363,  374. 

Sun  the  source  of  energy,  235. 

Symbols,  chemical.  157-166,  207; 
how  determined,  152,  278,  282. 

Synthesis,  110,  191;  of  alizarine, 
351 ;  of  organic  compounds,  323. 

Synthetical  reactions,  205-208. 

Table  of  alcohols,  340;  of  atomic 
weight  of  carbon,  143 ;  of  atomic 
weight  of  chlorine,  141 ;  of  atom- 
ic weight  of  hydrogen,  144 ;  of 
atomic  weight  of  oxygen,  141 ; 
of  calorific  power  of  combusti- 
bles, 213  ;  of  compounds  of  man- 
ganese and  fluorine,  132  ;  of  com- 
pounds of  nitrogen  and  oxygen, 
132;  of  dimensions  of  light- 
waves, 16 ;  of  elementary  sub- 
stances, 128;  of  heat  of  forma- 
tion, 374 ;  of  hydrides  of  methyl, 
ethyl,  etc.,  339  ;  law  of  multiple 
proportions,  133  ;  quanti valence 
of  atoms,  262,  266  ;  specific  heat 
of  elementary  substances,  148  ; 
thickness  of  soap-film,  24. 

Tartronic  acid,  342. 

Temperature,  39-46  ;  absolute  scale, 
41;  centigrade  scale,  40;  Fahr- 
enheit scale,  40. 

Tenacity  induced,  63. 

Test-papers,  182. 


Tetratomic  hydrates,  311. 

Thermo -chemistry,  principles  of, 
370. 

Thermometer,  40. 

Thomson,  Sir  William,  size  of  mol- 
ecules, 27. 

Toluidine,  346. 

Toluol,  345. 

Triatomic  hydrates,  811. 

Triplumbic  hydrate,  273. 

Trisodic  phosphate,  311. 

Turmeric-paper,  182. 

Unstable  products,  falling  back  of, 

357. 
Urea,  crystallization  of,  52 

Valeric  acid,  normal,  308  ;  isomeric 
modifications,  331. 

Vapors,  condition  of,  7  ;  interpene- 
tration  of,  9  ;  specific  gravity  of, 
78-85. 

Victor  Meyer's  method  for  vapor- 
density,  85. 

Water,  decomposed  by  electricity, 
103,106;  decomposed  by  sodium, 
276;  decomposed  by  potassium, 
281  ;  hardness  of,  180 ;  heat  of 
formation,  374  ;  influences  chemi- 
cal changes,  161 ;  synthesis  of, 
218. 

Waves  of  light,  16. 

Weight,  important  relations  in  chem- 
istry, 111 ;  of  molecules,  68,  73- 
85  ;  the  measure  of  mass,  91,  92. 

Weights  and  measures  (French  sys- 
tem), 69. 

W7 eights  of  atoms,  133. 

Wollastonite,  314. 

Zinc  sulphate,  288  ;  heat  of  forma- 
tion, 374. 


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"  A  work  which  constitutes  in  many  ways  the  most  instructive  review  that  has  ever 
been  written  of  the  evolution  of  human  knowledge  in  its  conflict  with  dogmatic  belief. 
...  As  a  contribution  to  the  literature  of  liberal  thought,  the  book  is  one  the  impor- 
tance of  which  can  not  be  easily  overrated." — Boston  Beacon. 

"  The  most  valuable  contribution  that  has  yet  been  made  to  the  history  of  the  con- 
flicts between  the  theologists  and  the  scientists." — Buffalo  Commercial. 

"  Undoubtedly  the  most  exhaustive  treatise  which  has  been  written  on  this  subject. 
„  .  .  Able,  scholarly,  critical,  impartial  in  tone  and  exhaustive  in  treatment."—  Boston 
Advertiser. 


New  York  :  D.  APPLETON  &  CO.,  72  Fifth  Avenue. 


D.   APPLhTON   AND  COMPANY'S  PUBLICATIONS. 

INVOLUTION    OF    MAN    AND    CHRISTIAN- 
J-^     ITY.     New  edition.     By  the  Rev.  HOWARD  MACQUEARY. 
With  a  new  Preface,  in  which  the  Author  answers  his  Critics,, 
and  with  some  important  Additions.     I2mo.     Cloth,  $1.75. 
"  This  is  a  revised  and  enlarged  edition  of  a  book  published  last  year.     The  authoi 
reviews  criticisms  upon  the  first  edition,  denies  that  he  rejects  the  doctrine  of  the  in- 
carnation, admits  his  doubts  of  the  physical  resurrection  of  Christ,  and  his  belief  in 
evolution.     The  volume  is  to  be  marked  as  one  of  the  most  profound  expressions  of  the 
modern  movement  toward  broader  theological  positions." — Brooklyn  'limes. 

4<  He  does  not  write  with  the  animus  of  the  destructive  school;  he  intends  to  be, 
and  honestly  believes  he  is,  doing  a  work  of  construction,  or  at  least  of  reconstruction. 
...  He  writes  with  manifest  earnestness  and  conviction,  and  in  a  style  which  is  always 
clear  and  energetic." — Churchman. 

TT I  STORY  OF    THE    CONFLICT  BETWEEN 
JLJL      RELIGION   AND    SCIENCE.     By  Dr.   JOHN   WILLIAM 
DRAPER.     i2mo.     Cloth,  $1.75. 

"The  key-note  to  this  volume  is  found  in  the  antagonism  between  the  progressive 
tendencies  of  the  human  mind  and  the  pretensions  of  ecclesiastical  authority,  as  de- 


A  CRITICAL  HISTORY  OF  FREE  THOUGHT 
<tl     IN  REFERENCE   TO   THE  CHRISTIAN  RELIGION. 
By  Rev.  Canon  ADAM  STOREY  FARRAR,  D.  D.,  F.  R.  S.,  etc. 
I2mo.     Cloth,  $2.00. 

"  A  conflict  might  naturally  be  anticipated  between  the  reasoning  faculties  of  man 
and  a  religion  which  claims  the  right,  on  superhuman  authority,  to  impose  limits  on 
the  field  or  manner  of  their  exercise.  It  is  the  chief  of  the  movements  of  free  thought 
which  it  is  my  purpose  to  describe,  in  their  historic  succession,  and  their  connection 
with  intellectual  causes  We  must  ascertain  the  facts,  discover  the  causes,  and  read 
the  moral." — The  Author. 

/CREATION  OR    EVOLUTION?     A  Philosophical 
v     Inquiry.     By  GEORGE  TICKNOR  CURTIS.    I2mo.     Cloth,  $2.00. 

"  A  treatise  on  the  great  question  of  Creation  or  Evolution  by  one  who  is  neither  a 
naturalist  nor  theologian,  and  who  does  not  profess  to  bring  to  the  discussion  a  special 
equipment  in  either  of  the  sciences  which  the  controversy  arrays  against  each  other, 
may  seem  strange  at  first  sight;  but  Mr.  Curtis  will  satisfy  the  reader,  before  many  pages 
have  been  turned,  that  he  has  a  substantial  contribution  to  make  to  the  debate,  and  that 
his  book  is  one  to  be  treated  with  respect.  His  part  is  to  anply  to  the  reasonings  of  the 
men  of  science  the  rigid  scrutiny  with  which  the  lawyer  is  accustomed  to  test  the  value 
and  pertinency  of  testimony,  and  the  legitimacy  of  inferences  from  established  facts." 
= — New  York  Tribune. 

"  Mr.  Curtis's  book  is  honorably  distinguished  from  ^  sadly  too  great  proportion  of 
treatises  which  profess  to  discuss  the  relation  of  scientific  theories  to  religion,  by  its 
author's  thorough  acquaintance  with  his  subject,  his  scrupulous  fairness,  and  remark* 
able  freedom  from  passion." — London  Literary  World. 

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PIONEERS  OF  SCIENCE  IN  AMERICA. 

Sketches  of  their  Lives  and  Scientific  Work.  Edited  and  re- 
vised by  WILLIAM  JAY  YOUMANS,  M.  D.  With  PortraitSc 
8vo.  Cloth,  $4.00. 

Impelled  solely  by  an  enthusiastic  love  of  Nature,  and  neither  asking 
nor  receiving  outside  aid,  these  early  workers  opened  the  way  and  initiated 
the  movement  through  which  American  science  has  reached  its  present  com- 
manding position.  This  book  gives  some  account  of  these  men,  their  early 
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a  single  exception  accompanied  with  a  well-authenticated  portrait. 

"  Fills  a  place  that  needed  filling,  and  is  likely  to  be  widely  read." — N.  Y.  SUM. 

"  It  is  certainly  a  useful  and  convenient  volume,  and  readable  too,  if  we  judge  cor- 
rectly of  the  degree  of  accuracy  of  the  whole  by  critical  examination  of  those  cases 
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to  us  that  the  handy  volume  is  specially  to  be  commended  for  setting  in  just  historical 
perspective  many  of  the  earlier  scientists  who  are  neither  very  generally  nor  very  well 
known." — New  York  Evening  Post. 

"  A  wonderfully  interesting  volume.  Many  a  young  man  will  find  it  fascinating. 
The  compilation  of  the  book  is  a  work  well  done,  well  worth  the  doing." — Philadelphia 
Press. 

"  One  of  the  most  valuable  books  which  we  have  received." — Boston  Advertiser. 

"  A  book  of  no  little  educational  value.  ...  An  extremely  valuable  work  of  refer- 
ence."— Boston  Beacon. 

"/ 

to  pro1 
to  hist 

"  A  biographical  history  of  science  in  America,  noteworthy  for  its  completeness  and 
scope.  .  .  .  All  of  the  sketches  are  excellently  prepared  and  unusually  interesting." — 
Chicago  Record. 

"One  of  the  most  valuable  contributions  to  American  literature  recently  made.  .  .  . 
The  pleasing  style  in  which  these  sketches  are  written,  the  plans  taken  to  secure  ac- 
curacy, and  the  information  conveyed,  combine  to  give  them  great  value  and  interest. 
No  better  or  more  inspiring  reading  could  be  placed  in  the  hands  of  an  intelligent  and 
aspiring  young  man." — New  York  Christian  Work. 

"  A  book  whose  interest  and  value  are  not  for  to-day  or  to-morrow,  but  for  indefinite 
time."  — Rochester  Herald. 

"  It  is  difficult  to  imagine  a  reader  of  ordinary  intelligence  who  would  not  be  enter- 
tained by  the  book.  .  .  .  Conciseness,  exactness,  urbanity  of  tone,  and  interestingness 
are  the  four  qualities  which  chiefly  impress  the  reader  of  these  sketches." — Buffalo 

Express. 

"Full  of  interesting  and  valuable  matter."—  The  Churchman. 


New  York:   D.  APPLETON  &  CO.,  72  Fifth  Avenue. 


D.  APPLETON  &  CO.'S  PUBLICATIONS. 

MODERN    SCIENCE    SERIES. 
Edited  by  Sir  JOHN  LUBBOCK,  Bart.,  F.  R.  S. 

CA  USE  OF  AN  ICE  AGE.  By  Sir  ROBERT 
BALL,  LL.  D.,  F.  R.  S.,  Royal  Astronomer  of  Ireland  ;  author 
of  "  Star  Land,"  "  The  Story  of  the  Sun,"  etc. 

"  Sir  Robert  Ball's  book  is,  as  a  matter  of  course,  admirably  written.    Though  but  a 
Small  one,  it  is  a  most  important  contribution  to  geology."  —  London  Saturday  Review. 
"  A  fascinating  subject,  cleverly  related  and  almost  colloquially  discussed."  —  Phila- 
delphia Public  Ledges. 


T 


'T^HE  HORSE:    A   Study  in   Natural    History.      By 
-*        WILLIAM  H.  FLOWER,  C.  B.,  Director  in  the  British  Natural 
History  Museum.     With  27  Illustrations. 

"  The  author  admits  that  there  are  3,800  separate  treatises  on  the  horse  already  pub- 
fished,  but  he  thinks  th,*i  he  can  add  something  to  the  amount  of  useful  information 
now  before  the  public,  and  that  something  not  heretofore  written  will  be  found  in  this 
book.  The  volume  gives  a  large  amount  of  information,  both  scientific  and  practical, 
on  the  noble  animal  of  which  it  treats."  —  New  York  Commercial  Advertiser. 


OAK  :   A  Study  in  Botany.     By  H.  MARSHALL 
WARD,  F.  R.  S.     With  53  Illustrations. 
"  From  the  acorn  to  the  timber  which  has  figured  so  gloriously  in  English  ships 
and  houses,  the  tree  is  fully  described,  and  all  its  living  and  preserved  beauties  and 
virtues,  in  nature  and  in  construction,  are  recounted  and  pictured."  —  Brooklyn  Eagle. 

CTHNOLOGY  IN  FOLKLORE.      By  GEORGE  L. 
-^—  '    GOMME,  F.  S.  A.,  President  of  the  Folklore  Society,  etc. 

"The  author  puts  forward  no  extravagant  assumptions,  and  the  method  he  points 
out  for  the  comparative  study  of  folklore  seems  to  promise  a  considerable  extension  of 
knowledge  as  to  prehistoric  times.  "  —  Independent. 

HE  LAWS  AND  PROPERTIES  OF  MAT- 
TER.  By  R.  T.  GLAZEBROOK,  F.  R.  S.,  Fellow  of  Trinity 
College,  Cambridge. 

"  It  is  astonishing  how  interesting  such  a  book  can  be  made  when  the  author  has  a 
perfect  mastery  of  his  subject,  as  Mr.  Glazebrook  has.  One  knows  nothing  of  the 
world  in  which  he  lives  until  he  has  obtained  some  insight  of  the  properties  of  matter 
as  explained  in  this  excellent  work."  —  Chicago  Herald. 


FAUNA  OF  THE  DEEP  SEA.  By  SYDNEY 
J.  HICKSON,  M.A.,  Fellow  of  Downing  College,  Cambridge. 
With  23  Illustrations. 

"That  realm  of  mystery  and  wonders  at  the  bottom  of  the  great  waters  is  gradually 
being  mapped  and  explored  and  studied  until  its  secrets  seem  no  longer  secrets.  .  .  . 
This  excellent  book  has  a  score  of  illustrations  and  a  careful  index  to  add  to  its  value, 
<and  in  every  way  is  to  be  commended  for  its  interest  and  its  scientific  merit."  —  Chicag* 
Times. 

Each,  lamo,  cloth,  $1.00. 

New  York:  D.  APPLETON  &  CO.,  72  Fifth  Avenue. 


